a tibial implant 1 of a knee prosthesis has a lateral condylar bearing surface 2 and a medial condylar bearing surface 3 . the bearing surfaces are basically concave . as fig3 shows , the lateral condylar bearing surface 2 comprises an end section 4 , which , in a top view of the tibial plateau , slopes down to the dorsal edge of the implant . in the embodiment under consideration here , the end section 4 is convexly curved . a transition section 5 between the concave part of the condylar bearing surface 2 and the end section 4 also has concave curvature . a broken line 6 indicates the normal course of the condylar bearing surface 2 without the downwardly sloping end section 4 . during bending of the knee , in which the femur carries out a rotation relative to an axis perpendicular to the tibial plateau , the downwardly sloping end section 4 aids luxation and prevents the femur or femoral implant from striking the dorsal edge of the tibial implant 1 . a femoral implant 7 shown in fig4 can be advantageously used together with the tibial implant 1 described with reference to fig1 to 3 . the femoral implant 7 has a lateral condylar component 8 and a medial condylar component 9 . an extension 10 with a contact surface 11 is connected to the dorsal end of the lateral condylar component 8 . the contact surface 11 is designed to be approximately complementary to the end section 4 of the condylar bearing surface 2 of the tibial implant 1 . during a flexion of the knee , the contact surface 11 of the extension 10 contacts the end section 4 . on the one hand , this produces leverage , and , on the other hand , the femoral implant 7 is exactly supported on the tibial implant 1 by the extension resting against it . both facilitate luxation , i . e ., the sliding of the condylar component 8 off the condylar bearing surface 2 . in the embodiments described below , parts that are the same or have the same function are designated by the same reference number as in the preceding drawings except that the letter a , b , c , d , e , or f is added to the given reference number . the tibial plateau of fig6 differs from the tibial plateau of fig1 in that a medial condylar bearing surface 3 a is formed as a depression , whose width increases in the dorsal direction as indicated by the contour line 12 . this shaping of the overall concave condylar bearing surface 3 a ensures that the contact area of the medial condyle of the femur or of the medial condylar component of a femoral implant during flexion , in which the femur rotates about an axis perpendicular to the tibial plateau , does not shift in the ventral direction but rather , as viewed in the ventral direction , remains approximately in the first third of the medial condylar bearing surface 3 a . besides a condylar bearing surface 3 b , which corresponds to the condylar bearing surface 3 a , the tibial implant 1 b shown in fig7 additionally has a dorsal extension 13 . the extension 13 forms part of a third condylar bearing surface 14 that extends between the condylar bearing surfaces 2 b and 3 b . this third bearing surface 14 ascends in the dorsal direction to the free end of the extension 13 and runs in the form of an arc that follows approximately a contour 15 of the medial condylar bearing surface 3 b . the tibial implant shown in fig7 and 8 can be used in a knee prosthesis together with the femoral implant 7 b shown in fig9 . in addition to a lateral condylar component 8 b and a medial condylar component 9 b , the femoral implant 7 b has a projection 16 between the condylar components . if , during an extension of the flexed knee , the femur is to move back into the rotational position corresponding to the unflexed knee , the return rotation of the femur associated with this requires that the lateral condyle 8 b slide up the end section 4 b to the concave part of the condylar bearing surface 2 b . this necessitates lifting of the femur , which is accomplished by virtue of the fact that during the extension , the projection 16 comes into contact with the third bearing surface 14 , which has a curvature in accordance with the necessary rotation of the femur . in the tibial implant 1 c shown in fig1 and 11 , a downwardly sloping section 4 c of a lateral condylar bearing surface 2 c makes a transition to another bearing surface 17 , which extends basically perpendicularly to the tibial plateau . the other bearing surface 17 is formed by an extension 18 that projects in the direction of the tibia . in the illustrated embodiment , the projecting length of the extension 18 decreases linearly towards the ventral edge of the tibial plateau 1 c . a milled - out recess for receiving an insert of this type can be produced during the implantation with comparatively little effort . alternatively , the additional bearing surface could also be formed by a projecting tongue , as is indicated in fig1 by a broken line . during the luxation , the femur makes contact with the additional bearing surface 17 . especially during prolonged flexions , such as occur while sitting on the floor , the implant protects the tibia from long - term stress by the luxated femur . fig1 shows additional forms of tibial implants with different condylar bearing surfaces 2 d , 2 d ′, and 2 d ″. the condylar bearing surface 2 d ″ is shaped convexly in the form of a circular arc . fig1 shows a top view of a tibial implant with condylar bearing surfaces 2 e and 3 e . the implant has a recess through which cruciate ligaments 22 and 23 can pass . fig1 shows additional tibial implants with condylar bearing surfaces 2 f and extensions 18 f . the extensions 18 f have various additional bearing surfaces 17 f , 17 f ″, 17 f ″′, and 17 f ″″.
the invention relates to a knee prosthesis with a tibial part that has concave or plane condylar bearing surfaces . according to the invention , the lateral condylar bearing surface merges into a preferably convex end portion that falls away in the dorsal direction .