Prosthetic joint

Abstract

A joint prosthesis comprising, e.g., a femoral component and a tibial component. The medial and lateral condylar articular surfaces may have substantially uniform and equal radii from full extension to about 90° of flexion. From 90°, the lateral condylar articular surface has a smaller radius than the medial condylar articular surface such that the medial condyle gradually becomes increasingly more proud than the lateral condyle to facilitate internal rotation of the tibia at deep flexion. Also, the tibial articular component may include a post intermediate the medial and lateral compartments that engages a cam on the femoral articular component between the medial and the lateral condylar articular surfaces. The cam and post become congruent at flexion angles of approximately 70° flexion and mate symmetrically during the first 20°-30° of further flexion, and then mate asymmetrically at greater degrees of flexion to force internal rotation of the tibia.

What is claimed is: 
     
         1 . A prosthesis for replacement of at least part of a knee joint comprising:
 a tibial articular component having a medial compartment defining a medial tibial articular surface and a lateral compartment defining a lateral tibial articular surface; and   a femoral articular component having a medial condylar articular surface and a lateral condylar articular surface for bearing on the medial tibial articular surface and the lateral tibial articular surface, respectively;   wherein the medial condylar articular surface and the lateral condylar articular surface are substantially symmetric with each other for portions of the medial and the lateral condylar articular surfaces corresponding to angles of less than approximately 90° flexion and are asymmetric for portions of the medial and the lateral condylar articular surfaces corresponding to angles of approximately 90° and greater angles of flexion.   
     
     
         2 . The prosthesis of  claim 1  wherein the medial tibial articular surface and the lateral tibial articular surface are substantially symmetric with each other. 
     
     
         3 . The prosthesis of  claim 1  wherein the lateral and medial condylar articular surfaces are of substantially constant radius for portions of the medial and the lateral condylar articular surfaces corresponding to angles of less than approximately 90° flexion. 
     
     
         4 . The prosthesis of  claim 3  wherein the medial condylar articular surface is of substantially constant radius for a portion of the medial condylar articular surface corresponding to substantially all of the range of motion of the prosthesis and the lateral condylar articular surface has a smaller radius than the radius of the medial condylar articular surface a portion of the lateral condylar articular surface corresponding to angles of greater than approximately 90° flexion. 
     
     
         5 . The prosthesis of  claim 4  wherein the medial condylar articular surface has a radius smaller than the constant radius for a portion of the medial condylar articular surface corresponding to the last approximately 40° of flexion of the range of motion of the prosthesis. 
     
     
         6 . the prosthesis of  claim 4  wherein the medial condylar articular surface is proud of the lateral condylar articular surface for all portions of the medial and the lateral condylar articular surfaces corresponding to angles of greater than approximately 90° flexion. 
     
     
         7 . The prosthesis of  claim 4  wherein the medial tibial articular surface and the lateral tibial articular surface are substantially torroidal in shape. 
     
     
         8 . The prosthesis of  claim 7  wherein the lateral tibial articular surface has a larger surface area and extends farther in the anterior direction than the medial tibial articular surface. 
     
     
         9 . The prosthesis of  claim 7  wherein the lateral femoral condylar articular surface is less congruent with the tibial lateral articular surface in the saggital plane than is the medial femoral condylar articulating surface with the medial tibial articular surface. 
     
     
         10 . The prosthesis of  claim 1  further comprising:
 a post on the tibial articular component between the medial compartment and the lateral compartment extending substantially cranially; and 
 a cam on the femoral articular component extending substantially in a medial-lateral direction between the lateral condylar articular surface and the medial condylar articular surface; 
 wherein one of the post and the cam is partially symmetric and partially asymmetric such that the interaction of the post and the cam during articulation of the femoral articular component relative to the tibial articular component does not bias any rotation of the tibial articular component relative to the femoral articular component at relative orientations corresponding to knee flexion of less than 60° of knee flexion and the interaction of the post and the cam during articulation of the femoral articular component relative to the tibial articular component biases internal rotation of the tibial articular component relative to the femoral articular component at relative orientations corresponding to knee flexion of greater than 110° of knee flexion. 
 
     
     
         11 . The prosthesis of  claim 10  wherein the cam has a bearing surface for bearing against the post and the asymmetry is on the cam bearing surface and the cam bearing surface is substantially catenoid shaped and the asymmetry comprises an enlargement of the cam bearing surface on a lateral side of the cam, the asymmetry located such that it becomes congruent with the post at angular orientations of the femoral articular component relative to the tibial articular component corresponding to angles of greater than 110° of knee flexion. 
     
     
         12 . The prosthesis of  claim 11  wherein the cam bearing surface comprises a symmetric portion and an asymmetric portion and is shaped and positioned on the femoral articular component relative to the post on the tibial articular component such that the cam and post do not contact each other at angular orientations of the femoral and tibial articular components to each other corresponding to knee flexion of less than 60°, the symmetric portion of the cam bearing surface first contacts the post at an angular orientation of the femoral and tibial articular components to each other corresponding to knee flexion of between 60° and 80°, the symmetric portion of the cam remains congruent with the post at angular orientations of the femoral and tibial articular components to each other corresponding to knee flexion for the first 20° to 30° of rotation after first contact, and the asymmetric portion of the cam bearing surface contacts the post at an angular orientation of the femoral and tibial articular components to each other corresponding to knee flexion of between 80° and 110° and remains congruent with the post for relative orientations of the femoral and tibial articular components corresponding to deeper angles of knee flexion. 
     
     
         13 . A prosthesis for replacement of at least part of a knee joint comprising:
 a tibial articular component having a medial compartment defining a medial tibial articular surface, a lateral compartment defining a lateral tibial articular surface, and a post between the medial compartment and the lateral compartment extending substantially cranially; and   a femoral articular component having a medial condylar articular surface or bearing on the medial tibial articular surface, a lateral condylar articular surface for bearing on the lateral tibial articular surface, and a cam extending substantially in a medial-lateral direction between the lateral condylar articular surface and the medial condylar articular surface;   wherein one of the post and the cam is partially symmetric and partially asymmetric such that the interaction of the post and the cam during articulation of the femoral articular component relative to the tibial articular component does not bias any rotation of the tibial articular component relative to the femoral articular component at relative orientations corresponding to knee flexion of less than 60° of knee flexion and the interaction of the post and the cam during articulation of the femoral articular component relative to the tibial articular component biases internal rotation of the tibial articular component relative to the femoral articular component at relative orientations corresponding to knee flexion of greater than 110° of knee flexion.   
     
     
         14 . The prosthesis of  claim 13  wherein the cam has a bearing surface for bearing against the post and the asymmetry is on the cam bearing surface and the cam bearing surface is substantially catenoid shaped and the asymmetry comprises an enlargement of the cam bearing surface on a lateral side of the cam, the asymmetry located such that it is congruent with the post at angular orientations of the femoral articular component relative to the tibial articular component corresponding to angles of greater than 110° of knee flexion. 
     
     
         15 . The prosthesis of  claim 14  wherein the asymmetry first becomes congruent with the post at an angular orientation of the femoral articular component relative to the tibial articular component corresponding to an angle of knee flexion in the range of about 80° to about 110°. 
     
     
         16 . The prosthesis of  claim 15  wherein the cam is positioned on the femoral articular component relative to the position of the post on the tibial articular component such that the cam and post do not contact each other at relative angular orientations corresponding to knee flexion of less than 60°. 
     
     
         17 . The prosthesis of  claim 15  wherein the cam is positioned on the femoral articular component relative to the position of the post on the tibial articular component such that the cam and post do not contact each other at relative angular orientations corresponding to knee flexion of less than 60°, contacts the post at relative angular orientations corresponding to knee flexion of between 60° and 80°, and remains in contact with the post at relative angular orientations corresponding to deeper knee flexion. 
     
     
         18 . The prosthesis of  claim 17  wherein the cam bearing surface comprises a symmetric portion and an asymmetric portion wherein the cam is shaped and positioned on the femoral articular component relative to the post on the tibial articular component such that the symmetric portion of the cam is congruent with the post at relative angles of orientation corresponding to the first 20° to 30° of knee flexion including and after first contact of the cam and the post, and the asymmetric portion of the cam is congruent with the post for relative angular orientations of the femoral articular component and the tibial articular component corresponding to deeper angles of knee flexion. 
     
     
         19 . The prosthesis of  claim 13  wherein the cam is located posteriorly of the post. 
     
     
         20 . The prosthesis of  claim 19  wherein the post comprises a posteriorly-facing bearing surface for the cam, the bearing surface comprising a segment of a cylindrical surface. 
     
     
         21 . A prosthesis for replacement of at least part of a knee joint comprising:
 a tibial articular component having a medial compartment defining a medial tibial articular surface, a lateral compartment defining a lateral tibial articular surface, and a first formation between the medial compartment and the lateral compartment; and   a femoral articular component having a medial condylar articular surface for bearing on the medial tibial articular surface, a lateral condylar articular surface for bearing on the lateral tibial articular surface, and a second formation between the lateral condylar articular surface and the medial condylar articular surface for bearing on the first formation of the tibial articular component;   wherein at least one of the first and second formations is partially symmetric and partially asymmetric such that the interaction of the first and second formations during articulation of the femoral articular component relative to the tibial articular component does not bias any rotation of the tibial articular component relative to the femoral articular component at relative orientations corresponding to knee flexion of less than 60° of knee flexion and the interaction of the femoral and tibial formations during articulation of the femoral articular component relative to the tibial articular component biases internal rotation of the tibial articular component relative to the femoral articular component at relative orientations corresponding to knee flexion of greater than 110° of knee flexion; and   wherein the first formation is a post extending substantially cranially and the second formation is a cam extending substantially in a medial-lateral direction and wherein the cam has a bearing surface for bearing against the post and the cam bearing surface is substantially catenoid shaped and the asymmetry comprises an enlargement of the cam bearing surface on a lateral side of the cam, the asymmetry located such that it is congruent with the post at angular orientations of the femoral articular component relative to the tibial articular component corresponding to angles of greater than 110° of knee flexion.