Prosthesis (i.e. – artificial body members) – parts thereof – or ai – Implantable prosthesis
Patent
1994-03-18
1995-10-10
Graysay, Tamara L.
Prosthesis (i.e., artificial body members), parts thereof, or ai
Implantable prosthesis
606 70, 606 71, A61F 202
Patent
active
054567190
DESCRIPTION:
BRIEF SUMMARY
With endoprostheses, a part made of viscoelastic synthetic resin such as polyethylene, which forms, for example, a joint surface of the endoprosthesis, often has to be fixed to a metal support part. This screw requires locking against undesired rotation in the release direction. In mechanical engineering, a screw lock is known (EP-A-0290967) which is based on the fact that the force-transmitting end face of the screw is provided with radial ribs which dig into the countersurface. The depth to which the ribs penetrate into the countersurface is determined by the force with which the screw is tightened. It can be limited by providing deepened contact surfaces between the ribs, which contact surfaces, at the intended penetration depth of the ribs, bear against the countersurface. However, the particular penetration depth of the ribs is dependent on the screwing force. If the screw is loose, the locking also no longer functions. In the case underlying the invention, on the other hand, only a slight screwing force, if any, normally acts on the synthetic resin part to be fastened; the known principle can therefore not be directly used here.
The invention therefore aims to find a screw lock which is adapted to these specific conditions. The solution according to the invention is provided by an assembly comprising a prosthesis part of viscoelastic synthetic resin fastened to a support part of the endoprosthesis by means of a screw element. The support part is more resistant to plastic displacement than the synthetic resin prosthesis part and is provided with an abutment while the screw element includes a head and a shank with a stop surface on the shank in spaced relationship to the head. The stop surface cooperates with the abutment to limit axial movement of the head relative to the prosthesis part and to seat the screw element. The seated position of the screw element is determined by means of the interacting stop surface on the screw element and the abutment on the support part. The head of the screw element has saw-toothed elevations facing the synthetic resin prosthesis part. The saw-toothed elevations are formed by tapered flanks forming leading surfaces during tightening of the screw element and steeper tooth flanks directed against the release direction of the screw element. In this way, the leading surfaces effect a viscoelastic displacement of the synthetic resin during tightening of the screw element, after which the displaced synthetic resin volume relaxes at least partly into the depressions formed between the elevations so that the steeper tooth flanks engage the relaxed surface of the synthetic resin prosthesis to resist loosening of the screw element.
By virtue of the fact that the relative position of the locking surface of the screw element on the one hand and of the surface of the synthetic resin part interacting therewith is determined by the design, the locking action is independent of the screwing force. It is also ensured that the synthetic resin part cannot be damaged by the fact that the locking surface is tightened with too great an assembly force. Rather, it is ensured under all assembly conditions that the synthetic resin material can only be plastically stressed to a predetermined degree.
The elevations and depressions are designed such that only a plastic material displacement of the synthetic resin occurs, which is subsequently at least partly eliminated again by relaxation. The relaxation provides a mutual interlocking of the locking surface with the associated synthetic resin surface, and thus the desired mutual locking action between the screw element and synthetic resin. The elevations or depressions of the screw element are preferably designed so as to be saw-toothed in the circumferential direction, their steeper tooth flanks being directed against the release direction.
If a screw is used for fastening, it may have a thread at its end, which interacts with a threaded bore in the metal support part, and may also have, adjacent to said thread at the head end, a thickened shank
REFERENCES:
patent: 1368896 (1921-02-01), Chambley
patent: 1813053 (1931-07-01), Hosking
patent: 3825051 (1974-07-01), Sigmund
patent: 4822366 (1989-04-01), Bolesky
Graysay Tamara L.
Waldemar Link (GmbH & Co)
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