the depicted illustrative embodiment of the prosthesis according to the invention is an ankle joint prosthesis . it will be noted that the invention can also be applied to other types of endoprostheses , for example intervertebral endoprostheses . the important point is that the endoprosthesis has two bearings whose planes of movement are defined by slide surfaces . the endoprosthesis according to the depicted illustrative embodiment basically comprises three components . the first component is a shin bone component 1 designed to be arranged on the lower end of a tibia 91 . it has a plate - shaped part 10 whose bottom forms a flat slide surface 11 . the shin bone component 1 is connected fixedly to the tibia 91 . the fibula 90 extends parallel to the tibia 91 . the prosthesis further comprises a component 4 which is connected to the ankle bone 92 . it has a saddle - like configuration and has a convexly curved slide surface 44 on its top . it can be configured in the manner of a jacket of a cylinder . however , it can equally well have a cone - shaped design . a guide rib 46 , which lies in the ap direction , is arranged on it . this serves for guiding purposes in a flexion and extension movement of the ankle joint . an intermediate part 2 is arranged between the shin bone component 1 and the ankle bone component 4 . on its top , it has a flat slide surface 21 which is configured to match the slide surface 11 of the shin bone component 1 . on its bottom , the intermediate part 2 has a slide surface 24 which is congruent to the slide surface 44 of the ankle bone component 4 . it additionally has a groove 26 which is designed to receive the rib 46 in a longitudinally displaceable manner . in this way , the intermediate part 2 is guided laterally in relation to the ankle bone component 4 . only flexion and extension movements are thus permitted . by contrast , the plane slide surfaces 11 , 21 permit any desired movement in a horizontal plane , that is to say both longitudinal and transverse movements and also , in particular , a rotation about the vertical axis . the shin bone component 1 and the ankle bone component 4 are expediently made of metal , for example a cobalt - chromium alloy provided on its respective outer face with a coating that promotes bone growth ( for example calcium phosphate ). the intermediate part 2 , by contrast , is preferably made of a plastic material that promotes sliding , in particular polyethylene . however , this is not intended to rule out the possibility of also using other materials with sufficient strength and slidability . fig1 shows the endoprosthesis at its intended site of implantation at the distal end of the tibia 91 . at its distal end , the tibia 91 forms a plateau on which the shin bone component 1 of the endoprosthesis according to the invention is arranged . this plateau is limited in the medial direction by a continuation of the tibia 91 , the so - called medial malleolus 93 , and in the lateral direction by a corresponding continuation of the fibula 90 , namely the lateral malleolus 94 . they enclose the plateau of the tibia 91 , and therefore the shin bone component 1 of the endoprosthesis , like a fork . this can be seen clearly in fig2 . it has been shown that , some time after implantation , a formation of tissue material ( fibrosis ) 99 often occurs in the area between the medial malleolus 93 and the intermediate part . this can cause pain which not only could be very unpleasant for the patient but in quite a few cases could also necessitate surgical intervention to remove the tissue material 99 . according to the invention , in order to avoid or reduce the fibrotic tissue material 99 , a projection 39 is formed at least on a longitudinal face 33 of the intermediate part 2 , expediently on the medial face . the projection extends outward relative to a contour which is congruent with the intermediate part 2 . the projection preferably has an arc - shaped outer contour , the arc extending across the entire length . the projection 39 is expediently curved in two dimensions , that is to say it has a spherical surface shape . the radii of curvature here are of different sizes , a weak curvature in the horizontal plane ( as is shown in fig2 ) and a stronger curvature in a frontal plane ( as is shown in fig1 ). to obtain the largest possible radius of curvature in the horizontal plane , the midpoint 30 of the circle defined by the radius of curvature preferably does not lie centrally in the intermediate part 2 but is instead eccentrically offset in the lateral direction and preferably also in the frontal direction . the outer face of the projection 39 is smooth . the illustrative embodiment shown represents one option , specifically one in which the front face and the rear face of the intermediate part 2 are also each provided with a projection 39 ′ and 39 ″, respectively . as can be seen from fig3 , they deviate in shape from the projection 39 , namely being of cylindrical - sleeve shape . in addition , they do not extend over the entire height of the intermediate part 2 . a transition of equal curvature between the projections 39 , 39 ′, 39 ″ is not necessary , but the geometries are expediently chosen such that the transition is stepless . the lateral longitudinal face of the intermediate part 2 expediently has no projection . this serves to ensure free movement of the intermediate part 2 . this also has the advantage of providing an unambiguous orientation of the intermediate part 2 , as a result of which the danger of its being fitted in an incorrect position is reduced . the projection 39 is normally designed in one piece with the intermediate part 2 . however , this should not rule out the possibility of choosing a multi - part construction in which the projection 39 is designed as a separate part and is secured on the intermediate part 2 by suitable securing means . the latter option affords the advantage that , for the projection 39 , it is possible to choose a material which especially promotes sliding and is especially suitable for contact with the fibrotic tissue material 99 , without compromising the mechanical load - bearing capacity of the intermediate part 2 . in the areas where contact with tissue material 99 is to be expected , the projection 39 is provided with a protective layer 50 . by this means , there is no danger of damage to the surrounding tissue 99 by abrasion or the like . during movement of the endoprosthesis , particularly during rotation , but also during a forward or rearward movement in the longitudinal direction , the projection 39 ensures that fibrotic tissue material 99 is forced back . this effectively counteracts infiltration of the fibrotic tissue 99 into the area of the endoprosthesis . finally , it will be noted that the configuration according to the invention of an intermediate part 2 with a projection 39 is not limited to ankle - joint endoprostheses .
the invention relates to an endoprosthesis for replacement of a joint , in which slide surfaces that have different contours and correspondingly define movement planes for bearing are formed by an intermediate part . according to the invention , the intermediate part has a projection , preferably a convex projection , on at least one side face , in order to avoid fibrosis .