Design of Kinemax total knee replacement. Bearing surfaces

Wear and deformation of the plastic patella component and the tibial bearing surface are recognised concerns in condylar knee designs. This paper discusses an approach to minimising the wear and deformation, while allowing sufficient laxity for activities of everyday living. The femoral component was designed from an average geometry based on sectioning of whole knees. The patella groove of the femoral component has an anatomical profile. The patella component is a close fit in the skyline view, providing a wide area of contact. The laxity requirements were satisfied, based on a 3-D motion study. For the tibial-femoral surfaces, 3-D motion studies showed that plus/minus 12 degrees rotation were needed about neutral, and 13 mm of A-P displacement. A computer program was written to generate the tibial surfaces which allowed for these motions. The contact stresses were considered acceptable for low wear. However, if interchangeability was not required, the stresses could be reduced further. Finally, if the required motion was reduced, especially A-P displacement, stresses could be reduced by over 24 percent. Such a surface was designed for use in heavy, young and active patients.