Multi-axial simulator testing is a standard tool in development and evaluation of artificial hip and knee joints. Close to in vivo conditions are desirable in terms of implant set-up, input parameters, and environment. Thus, specimens are subjected to loads and displacements measured at patients with artificial joints, and a lubrication medium is provided to simulate human synovial fluid. Implant wear is then commonly assessed through periodic gravimetric measurements. However, ultra-high weight polyethylene (UMWPE), the typical material for tibial plateaus, has been shown to gain weight through fluid absorption, which influences the results of such tests. A recent study performed on a 4-DOF knee joint simulator points out that composition variations in the lubricating medium highly influences fluid uptake of UHMWPE. Two sera with different additives have been compared, and showed significant deviations in weight measurements. Wear readings have to be corrected accordingly. Further implications, e.g. alteration of the friction coefficient, have to be considered.