Rotating platform mobile bearing knees are an appealing approach to the problems of tibial loosening and rotational malignment in fixed bearing knees. A potential disadvantage is the additional large articular surface that accommodates tibio-femoral rotation. Accurately assessing the tribological performance of this additional articular surface is important to understanding how mobile bearings perform in terms of generating polyethylene wear debris and associated osteolysis. A series of 76 retrieved Sigma Rotating Platform bearings were assessed for damage rating according to conventional protocol and through-thickness wear measurements were taken. The results show that the rotation surface of these bearings is very commonly subject to moderate and severe damage and that damage can occur early following implantation. The decrease of the through-thickness dimension is strongly correlated with time in vivo. The rotation surface damage rating shows a weak though statistically significant correlation to through-thickness wear. Three dimensional surface profilometry on the bearings illuminates phenomena that can explain the paradoxical observations that severely damaged bearings may not be worn and worn bearing areas are smoother and show less damage than unworn areas. This study finds that bearing damage is distinct from bearing wear and the two terms are not interchangeable in the context of assessing material loss from artifi-cial knee bearings. While both processes are important in the tribology of knee devices, it is not accurate to use damage on ultrahigh molecular weight polyethylene as a proxy for wear.