Knee prosthesis

Abstract

A knee prosthesis comprises a femoral component for securement to a femur, the femoral component defining medial and lateral condyles and an intercondylar groove; and a tibial component for securement to a tibia, the tibial component having respective bearing surfaces shaped to engage with said condyles both when the knee, in use, is extended and also over a range of flexion. The femoral and tibial components together comprise interengaging means, for stabilising against antero-posterior relative movement of the femoral and tibial components. One of the condyles and the respective bearing surface of the tibial component are at least partially congruent, at least at angles of flexion below a predetermined angle. The other bearing surface is incongruent with the other condyle in at least the antero-posterior direction so as to allow for antero-posterior movement of the femoral component relative to the bearing surfaces of the tibial component.

1 . A knee prosthesis comprising:
 a femoral component for securement to a femur, the femoral component defining medial and lateral condyles and an intercondylar groove; and   a tibial component for securement to a tibia, the tibial component having respective bearing surfaces shaped to engage with said condyles both when the knee, in use, is extended and also over a range of flexion, wherein:   the femoral and tibial components together comprise interengaging means, for stabilising against antero-posterior relative movement of the femoral and tibial components at flexion above a first predetermined angle;   one of the condyles and the respective bearing surface of the tibial component are at least partially congruent, through at least a plane extending through a contact line between said one condyle and the respective bearing surface, at least at angles of flexion below a second predetermined angle; and   the other bearing surface is incongruent with the other condyle in at least the antero-posterior direction so as to allow for antero-posterior movement of the femoral component relative to the bearing surfaces of the tibial component.   
     
     
         2 . A knee prosthesis according to  claim 1 , wherein the interengaging means comprises a peg and cooperating cam. 
     
     
         3 . A knee prosthesis according to  claim 2 , wherein the cam is formed in the region of the intercondylar groove of the femoral component and the peg is formed projecting from the tibial component. 
     
     
         4 . A knee prosthesis according to  claim 1 , wherein the cam extends across the intercondylar groove from the medial side to the lateral side of the prosthesis. 
     
     
         5 . A knee prosthesis according to  claim 2 , wherein a surface of the peg which engages with the cam is rounded so as to allow the surface of the cam to rotate about the peg in a plane substantially parallel to a notional plane passing through both bearing surfaces of the tibial component. 
     
     
         6 . A knee prosthesis according to  claim 2 , wherein the knee prosthesis is rotatable from a first position in which said one condyle and respective bearing surface are congruent to a second position in which said one condyle and corresponding bearing surface become incongruent, the cam and peg engaging on rotation of the knee prosthesis after leaving the first position, and before when, or after reaching the second position. 
     
     
         7 . A knee prosthesis according to  claim 1 , wherein the first predetermined angle is in a range between approximately 50 and 120 degrees. 
     
     
         8 . A knee prosthesis according to  claim 1 , wherein the second predetermined angle is in a range between approximately 50 and 120 degrees. 
     
     
         9 . A knee prosthesis according to  claim 1 , wherein the medial condyle and corresponding bearing surface are congruent at least at angles of flexion below the second predetermined angle. 
     
     
         10 . A knee prosthesis according to  claim 1 , wherein the surface of the medial condyle and corresponding bearing surface are rounded with equal radius in both the flexion and extension facets of the prosthesis. 
     
     
         11 . A knee prosthesis according to  claim 1  wherein at least one of the condyles has point or line contact, as opposed to area contact, with a respective bearing surface. 
     
     
         12 . A knee prosthesis according to  claim 1  wherein at least the bearing surfaces of the tibial component comprise a cross-linked polymer. 
     
     
         13 . A knee prosthesis according to  claim 1  wherein the condyles are shaped in the manner of the threads on a screw, and the respective bearing surface is shaped in the manner of threads cut in a complementary nut. 
     
     
         14 . A knee prosthesis according to  claim 1  wherein at least one of the medial and lateral condyles are part-helical in shape. 
     
     
         15 . A knee prosthesis according to  claim 1 , wherein the other condyle is non-helical. 
     
     
         16 . A knee prosthesis according to  claim 1 , wherein the medial condyle is part-helical in shape and is configured for congruent contact with a respective bearing surface which is correspondingly part-helical in both the medio-lateral direction and the antero-posterior direction. 
     
     
         17 . A knee prosthesis according to  claim 16 , wherein the lateral condyle is part-helical in shape and the respective bearing surface for the lateral condyle is configured for incongruent contact in at least the antero-posterior direction. 
     
     
         18 . A knee prosthesis according to  claim 16 , wherein the lateral condyle is non-helical and the respective bearing surface is configured for incongruent contact in both the antero-posterior direction and the medio-lateral direction. 
     
     
         19 . A knee prosthesis according to  claim 1 , wherein the transverse cross-section of one condyle is flat, curved or semi-spherical. 
     
     
         20 . A knee prosthesis according to  claim 1 , wherein the transverse cross-section is different for each condyle.