Nowadays wear is recognized as one of the main concern of metal-on-metal (MoM) hip implants, causing osteolysis and the release of dangerous metallic ions. Numerical wear simulations of hip implants are an attractive tool to investigate and predict long-term wear at low cost. A few wear models have been proposed in the literature for MoM bearings [1-3], all based on the Archard wear law, as adhesion and abrasion are considered the main wear mechanisms. The reliability of such models mainly depends on a dimensional wear factor k whose evaluation/choice is actually a critical issue. Indeed k depends on many factors, such as lubrication regime, bearing materials and geometry, loading and kinematic conditions and thus can vary during a wear test/implant lifetime. This complex scenario is simplified in wear simulations which typically assume two constant values of k - equal for head and cup - one higher for the initial running-in phase (kri) and the other lower for the steady state phase (kss), according to experimental observations (Fig. 1). Such k values are generally estimated by matching numerical and experimental wear volumes obtained by hip joint simulator tests [2-3], sometimes even simulating conditions different from the experimental ones [3]. For these reasons and because of a complete neglect of the comparison between numerical and experimental wear maps, the validity of the wear models can be disputable
