Optimization of the bearing surface geometry of total knees

Various design criteria were examined in combination to find the ideal geometry for a condylar knee replacement. The criteria were the contact stresses on the plastic, femoral-tibial size interchangeability, patella lever arm, laxity and stability and the amount of bone resection required. The variables were the radii of curvature of the femoral and tibial bearing surfaces in the sagittal and frontal planes. Metal toroidal indentors were loaded onto dished surfaces of UHMWPE covering a range of radii and the contact areas measured. Using elasticity equations, the apparent elastic modulus of UHMWPE ranged from 400 to 600 MPa for less conforming to closely conforming surfaces. Using a value of 600 MPa, contact stresses were predicted for a complete spectrum of radii of curvature. Finite element analysis was used to determine the stresses beneath the contact patches when different femoral-tibial sizes were interchanged. A computergraphics program was written to analyse the effects of flexion, rotation and femoral roll-back on the contact point locations. An influential variable was the sagittal curvature of the femoral component, notably the point of transition between the posterior curve of small radius and the distal curve of larger radius. This affected the patella lever arm, the stability, and the bone resection. Interchangeability was primarily dependent upon the relative frontal radii. Contact stresses and contact locations depended upon the combination of sagittal and frontal radii. The most suitable geometrical combinations overall were discussed.