A COMPARISON OF CONTACT PRESSURES IN TIBIAL AND PATELLAR TOTAL KNEE COMPONENTS BEFORE AND AFTER SERVICE IN-VIVO

Laboratory testing of total knee components indicates that many design s produce contact stresses that exceed the yield strength of ultra-hig h molecular weight polyethylenes (UHMWPEs). It is often assumed that t he polyethylene component will creep and wear to become more conformin g over time, thus reducing these stresses. To test this theory, retrie ved polyethylene tibial and patellar components, which showed signs of increased contact area through in vivo deformation, were tested for c ontact stress against matching components using Fuji Prescale pressure -sensitive film. The results showed an inverse relationship between in itial conformity and in vivo changes in contact stress. More conformin g devices showed little or no change in contact stress, and less confo rming components showed small decreases in contact stress as a result of creep and wear. Even with these changes, however, the contact stres ses for nonconforming designs remained well above those for the more c onforming devices as well as the uniaxial yield strength of UHMWPE.