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