## 4 Toward the In Silico Wear Test  
In vitro studies play an important role across all fields of medical research, thus biomedical manufacturers have started with computer modelling and simulation. In this way they accelerate their product development processes and reduce the huge cost of bringing a new device to the market. Today, to maintain the exponential growth of innovation, the medical and pharmaceutical worlds are entering an era in which a growing number of experiments will be done on the computer to complete and accelerate in vivo and in vitro approaches. This has the potential to revolutionize science and medicine. Advanced computer modelling techniques (often referred to as in silico models), is an expression used to mean \"performed on computer or via computer simulation\" [38]. The expression in silico \"appears\" for the first time in 1989 in the workshop \"Cellular Automata: Theory and Applications\" in Los Alamos, New Mexico [39; 40]. The experimental tribological devices for the simulation of hip and knee prostheses have been improved over the years in order to make them able to reproduce tribological wear tests in kinematic and dynamic conditions very close to the real ones [30; 31; 32; 33]. The new trend of an in silico approach to the evaluation of the articular prostheses' wear represents, nowadays,  
Figure 3: Schematic representation of the so-called knee and hip joint simulators. In **(A)** a schematic station of a knee joint simulator; in **(B)** a schematic station of a hip joint simulator. This picture is a general view for joint simulators and not a representation of displacement or force control, or anatomical or upside-down positioning of the prostheses. a fascinating scientific challenge, which involves many disciplinary fields and which requires a deep collaboration between scientists from different areas [41; 42; 43; 44]. One of the most current in silico approaches is represented in Figure 4 with reference to the total hip replacement. With the aim to calculate the hip wear, taking into account the complex tribological phenomena acting in the artificial synovial joints, the proposed approach follows the scheme in Figure 4.  
The in silico procedure starts by evaluating the human motion kinematics in the framework of inverse dynamic analysis (motion capture) with reference both to normal gait and other desired daily activities [32; 45]. The obtained data are used for the calculation of the unsteady joint forces which are used as load conditions in joint Finite Element Analysis (FEM) [42; 43; 44]. The resulting stress-strain behavior of the artificial coupling have to be joined with the lubrication model for taking into account the complex synovial phenomena acting in the joint [41; 46]. Once the lubricating film is modelled, suitable wear models will furnish the possibility to obtain the in silico wear values for the investigated artificial joint. Of course this fascinating approach needs more accurate mathematical models in order to depict in an accurate way the real phenomena acting in the considered tribo-system. Many models which were proposed in literature are from a synovial lubrication point of view; in these models a combination of Boundary Lubrication (BL), Mixed Lubrication (ML), Hydrodynamic Lubrication (HL), and Elasto-Hydrodynamic Lubrication (EHL), are considered [47; 48]. The conditions of high loads and low relative motion could lead to a lubricated contact between the articulated surfaces despite their deformation, so that in these contact areas the contact pressure rises and Mixed Elasto-Hydrodynamic Lubrication (MEHL) occurs. Since the gap thickness values reached in this lubrication model are very low, the lubricant viscosity could be affected by the high pressure developed in the gap, so appropriate relationships between viscosity and pressure are necessary in the modelling of the non-Newtonian synovial fluid behavior [49]. The Archard wear equation [50] represents a simple and effective model for the description and calculation of wear.