Prosthesis (i.e. – artificial body members) – parts thereof – or ai – Implantable prosthesis – Bone
Patent
1992-04-23
1994-04-26
Isabella, David
Prosthesis (i.e., artificial body members), parts thereof, or ai
Implantable prosthesis
Bone
606 61, A61F 244
Patent
active
053063082
DESCRIPTION:
BRIEF SUMMARY
SUMMARY OF THE INVENTION
The invention relates to a intervertebral implant comprising a disc made of rigid material and having two opposing sides bordering respectively adjacent vertebrae. Each of the opposing sides includes a circular frontal area and a raised dome at the central portion thereof. The opposing sides further include roof shaped projections surrounding the raised dome, each of these projections having a pair of end faces, and a ridge edge and a pair of base edges extending between the end faces. The base edges and the ridge edge of these projections form respective areas of concentric circles.
German Offenlegungsschrift (published patent application) 28 04 936 and German Offenlegungsschrift (published patent application) 22 63 842, for instance, describe such a known endoprosthesis.
A drawback with such known endoprostheses is that in order to anchor them firmly to the vertebra either a complicated endoprosthesis shape and a corresponding milling of the bone or the use of bone cement is necessary.
The achievable protection against rotation or sideward dislocation is generally problematic due to the large mechanical strain exerted on the spine.
It is therefore an object of the invention to provide an endoprothesis of the intervertebral disc which provides a good and long-lasting fit between the vertebrae and with which micromovements and in particular rotations and sideward dislocation can be prevented.
The invention is based on the realization that the sideward dislocation of an intervertebral implant can be reliably prevented by a number of geometrical shaping measures whereby this shaping, in the case of a round and essentially disc-shaped implant, is such that the stops designed to prevent any movement in a radial and tangential direction are in the form of areas which are essentially oriented in a direction transverse to these radial and tangential directions of movement. The corresponding shaping is in addition such that the parts incorporated in the areas can penetrate the neighbouring vertebrae and are therefore shaped as roof-shaped projections. This penetration increases with the amount of loading applied so that the stopping effect also increases. The surface is also such that it can grow into the vertebrae which have been milled accordingly. The sideward dislocation or rotation of the intervertebral implant must be prevented by the geometrical shaping for at least as long as the implant has started to grow onto the bone.
According to the invention the frontal areas of the endoprosthesis bordering the vertebrae are circular and each comprise a central raised dome and roof-shaped projections whose longitudinal base edges form concentric parts of arcs of circles. The raised dome enables the endoprosthesis to be centered relative to the vertebrae and also helps to stabilize it with regard to sideward dislocations. However, dislocations and rotational movements are mainly prevented from occuring by the projections.
In an advantageous embodiment of the invention both the size and the shape of the raised dome are formed in accordance with the anatomical conditions of the vertebrae, in order that the raised dome can then be firmly pressed into the inner softer tissue of the vertebrae. The raised dome is then preferably convex and spherical in shape.
The roof-shaped projections can be pressed into the spongiosa of the vertebrae so that only one level cut must be carried out on the vertebra. The ridges of the roof-shaped projections preferably form, as do the longitudinal base edges, concentric parts of arcs of circles whereby their length is smaller than the length of the longitudinal base edges. Due to this and due to the centered position of the ridge relative to the projection area the gable faces are shaped as sloped, upwardly projecting triangles, which not only facilitates the pressing in of the implant into the bone but also facilitates the growing of the implant onto the bone.
The arrangement of the projections is preferably symmetrical so that the frontal area seems to consist of
REFERENCES:
patent: 4863477 (1989-09-01), Monson
patent: 4865603 (1989-09-01), Noiles
patent: 5015247 (1991-05-01), Michelson
Fuhrmann Gerhard
Gross Ulrich
Kaden Bertram
Schmitz Herman-Josef
Brittingham Debra S.
Isabella David
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