Quantification of Wear Rates and Plastic Deformation on Mobile Unicompartmental UHMWPE Tibial Knee Inserts

Experimental wear testing is an essential step in the evaluation of unicompartmental knee prostheses; the major mechanisms that dominate the wear of conventional ultra high molecular weight polyethylene tibial knee menisci are the sub-surface cracking and delamination that induce particle release by abrasion/adhesion and subsequently periprosthetic osteolysis. The aim of this study was to determine whether plastic deformation affects the wear of the polymer and to measure the magnitude of these effects. Wear test was performed using a displacement-control knee wear simulator with "three-plus-one" stations, in accordance with the ISO 14243-3/2. A state-of-the-art coordinate measuring machine was used to quantify the volumetric mass loss of the mobile knee polyethylene menisci as well as creep/plastic deformations. The volumetric wear measured by this method was compared to that measured by the gravimetric method. Raman spectroscopy showed morphology changes induced by mechanical stress in both the upper and lower surfaces of the menisci. The amorphous content increased at expenses of the crystalline orthorhombic content, which generally decreased in all menisci. A slight orthorhombic -> monoclinic phase transformation occurred upon mechanical stress. Plastic deformation appeared as the main factor affecting the trend of the spectroscopic markers and thus the morphology degradation.