Endoprosthesis, in particular knee joint prosthesis

The invention relates to an inlay which is suitable for an endoprosthesis, in particular a joint prosthesis. This inlay essentially comprises a flexible pad 5, a first flexible membrane 2 resting on the pad, and a second membrane 3, extending parallel to and above said membrane, in which support elements 5 are clipped. The support elements 4 rest with their undersides 4e on the first membrane 2, and with their surfaces 4a form a sliding and bearing surface for the joint head which is to be connected to the bone. The pad 5, composed of a liquid or gel-like medium, is situated in a well 1, and the two membranes 2 and 3 are fixed to the lateral walls 1a of the well by means of screws 6 and nuts 7.

The invention relates to a joint prosthesis.

Such joint prostheses, referred to as artificial joints, are intended to replace a damaged or diseased joint, for example a knee joint.

In the case of the knee joint, joint prostheses are composed of metallic joint heads having an operative connection with the tibia and the femur. An inlay made of polyethylene, for example, is provided between the joint heads which prevents direct metal contact and partially compensates for the incongruence of the joint heads forming the upper and lower sections.

Because of the significant continuous strain which occurs essentially in places, the design of this inlay is problematic, resulting in cold flow, delamination, and abrasion of the inlay material.

An inlay has been proposed in the invention published in WO 99/42061, comprising a flexible pad, a flexible membrane resting on the pad, and support elements provided on the membrane whose surface is used for supporting the joint heads. Such an inlay reduces the frictional force and lessens the abrasion and introduction thereof into the tissue surrounding the joint, which can result in malfunction and damage.

For this design of the inlay, the connection of the support elements to the membrane is not detached. On the other hand, studies have shown that adhesive connections do not permanently withstand the shear forces which occur during the rolling motion of the joint heads.

The object of the present invention, therefore, is to provide a joint prosthesis having support heads which are permanently fixed without impairing the flexibility.

This object is achieved according to the present invention by means of a second membrane extending above and parallel to the first membrane, and having retaining openings in which the support elements are anchored and which laterally support one another and with their undersides rest on the first membrane, both membranes being attached to the base body.

The support elements rest on the first membrane and are connected to the adjacent support elements exclusively by means of the second membrane. The support element thus has limited mobility perpendicular to the membrane, the support elements resting laterally on the adjacent support elements. In an exemplary embodiment, the support elements located at the edges are able to rest on the lateral walls of the well, which acts as the base body and which absorbs the resulting shear forces so that the second membrane which is used for retaining the support element is not subjected to shearing.

Other exemplary embodiments illustrate a specialized design of the support elements.

Still other exemplary embodiments illustrate proposals for attaching the membrane to the base body, for selecting the material of the membranes and the support elements, and for the design of the pad.

Silicone in particular has proven to be a satisfactory material for the membrane due to its great flexibility.

An exemplary design according to yet another embodiment of the invention is particularly suited for knee joint prostheses.

FIG. 1illustrates an inlay designed according to the invention which normally is situated between the joint heads, generally made of steel, of the joint prosthesis, which in turn is joined by screws, pins, or with cement to the bone, and in the case of a knee joint prosthesis, is joined to the tibia or femur.

The inlay essentially comprises a flexible but noncompressible pad5on which a first membrane2, and thereabove a second membrane3together with support elements4anchored therein, are situated. The membranes2and3are made of silicone, and the support elements4are made of polyethylene. The pad5is situated in a base body1, designed as a well and likewise made of polyethylene, having lateral walls1aextending to the surface4aof the support elements4. The membranes2and3are fixed to these walls1aby means of screws6and nuts7.

The surfaces4aof the support elements4and1bof the walls1aof the well form a slightly concave bearing surface for the joint head (not illustrated), which has a convex surface.

As shown most clearly inFIG. 3, the support elements4which have a double T-shaped cross section are divided by a constriction4binto an upper part4cand a base part4d. At their base part4dthe support elements are clipped and thus fixed in corresponding retaining openings in the membrane2. The underside4eof the base part is supported on the top side of the membrane2. The support elements are adjacently aligned, with the outer support elements4being supported by the lateral walls1aof the well1.

As shown in the top view according toFIG. 2, the support elements have a hexagonal cross section, thus resulting in good support on all sides.

As a result of the measure according to the invention, the support elements4are supported in the transverse direction so that the membrane used for retaining the support elements is not subjected to shearing during movement of the joint.

The axially acting pressure forces are absorbed by the pad5, which is composed of a fluid with little or no compressibility, preferably a saline solution or gel-like substance, provided in the well1and delimited from above by the membrane2. This pad5provides uniform distribution of the hydrostatic pressure.

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