Abstract:
The invention relates to a knee prosthesis with a tibial part ( 1 ) that has concave or plane condylar bearing surfaces ( 2, 3 ). According to the invention, the lateral condylar bearing surface ( 2 ) merges into a preferably convex end portion ( 4 ) that falls away in the dorsal direction.

Description:
BACKGROUND OF THE INVENTION 
     The invention concerns a knee prosthesis with a tibial implant that has concave or planar condylar bearing surfaces. 
     Tibial implants of this type are well known by use. They are sometimes used in conjunction with femoral implants, which have a medial condylar component and a lateral condylar component corresponding to the bearing surfaces of the tibial implant. 
     SUMMARY OF THE INVENTION 
     The objective of the invention is to create a new knee prosthesis of the aforementioned type that is improved in its functional capabilities, especially with respect to flexions of the knee. 
     The knee prosthesis of the invention which solves this problem is characterized by the fact that the lateral condylar bearing surface has an end section that slopes away in the dorsal direction. 
     The advantage of this is that the downwardly sloping end section of the lateral condylar bearing surface allows trouble-free luxation during knee flexion, which is associated with, among other things, a rotation of the femur about an axis perpendicular to the tibial plateau. The lateral femoral condyle or the lateral femoral component of the knee prosthesis can slide off the lateral bearing surface of the tibial implant. In conventional knee prostheses, the tibial and femoral components of the prosthesis come into contact with each other when the knee is bent, which limits the angle of flexion. 
     The lateral condylar bearing surface can make the transition to the downwardly sloping end section via a convexly curved transition section, and the end section itself is preferably convexly curved. 
     The end section extends in the dorsal direction, preferably over about one fifth to one third of the length of the lateral condylar bearing surface. 
     In another embodiment of the invention, the lateral condylar component of a femoral implant of the knee prosthesis has an extension, which comes into contact with the sloping end section during flexion of the knee. Advantageously, this extension forms both a lever and a guide, which facilitate the sliding of the lateral condylar component off the condylar bearing surface. 
     This extension preferably has a contact surface that is complementary to the end section. 
     In another embodiment of the invention, the medial condylar bearing surface can be shaped as a depression that widens in the dorsal direction. The advantage of this is that this depression ensures that during flexion of the knee, the contact area between the condylar bearing surface and the medial condyle does not experience an undesired shift in the ventral direction. 
     In another embodiment of the invention, an elevation with a third bearing surface is formed between the condylar bearing surfaces in the dorsal half of the implant. A projection formed between the condylar components of a femoral implant can be supported on this third bearing surface. Advantageously, this support assists the return rotation of the femur relative to the tibial implant during a flexion of the knee by facilitating the upward movement of the femur implant up the downwardly sloping end section that is necessary during the return movement. 
     This third bearing surface can be partly formed by a dorsal extension on the tibial implant and preferably ascends towards the free end of the dorsal extension. Corresponding to the rotation of the femoral implant relative to the tibial implant, the third bearing surface preferably runs in a curve that follows the contour of the medial condylar bearing surface. 
     In another embodiment of the invention, the downwardly sloping end section of the lateral condylar bearing surface makes a transition to another bearing surface that extends perpendicularly to the tibial plateau. An advantage here is that the luxated femur does not lie against the tibia but rather against the implant. 
     During prolonged knee flexion, e.g., while sitting on the floor with the legs crossed or similar positions, stresses on the tibia are avoided. 
     The invention is explained in greater detail below with reference to the specific embodiments of the invention that are illustrated in the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWING 
         FIG. 1  is a top view of the plateau of a tibial implant of the invention. 
         FIG. 2  shows the tibial implant of  FIG. 1  along sectional line C-C. 
         FIG. 3  shows the tibial implant of  FIG. 1  along sectional line A-A. 
         FIG. 4  is a side view of a femoral implant that can be used together with the tibial implant of  FIGS. 1 to 3 . 
         FIG. 5  shows the femoral implant of  FIG. 4  as viewed in the dorsal direction. 
         FIGS. 6 and 7  show additional embodiments of a tibial implant of the invention in top views of the implant. 
         FIG. 8  shows the tibial implant of  FIG. 7  as viewed in the ventral direction. 
         FIG. 9  shows a femoral implant that can be used with the tibial implant of  FIGS. 7 and 8  in a knee prosthesis as viewed in the ventral direction. 
         FIG. 10  is a side view of another embodiment of a tibial implant of the invention as viewed in the ventral direction. 
         FIG. 11  shows the tibial implant of  FIG. 10  along, sectional line B-B. 
         FIGS. 12 to 14  show further embodiments of tibial implants of the invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     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  FIG. 3  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  FIG. 4  can be advantageously used together with the tibial implant  1  described with reference to  FIGS. 1 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  FIG. 6  differs from the tibial plateau of  FIG. 1  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  FIG. 7  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  FIGS. 7 and 8  can be used in a knee prosthesis together with the femoral implant  7   b  shown in  FIG. 9 . 
     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  FIGS. 10 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  FIG. 11  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. 
       FIG. 12  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. 
       FIG. 13  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. 
       FIG. 14  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″″.