Synovial Lubrication Modeling of Total Hip Replacements Using Musculoskeletal Multibody Dynamics

Nowadays the biotribology of the human articulations is a challenging topic in the framework of biomechanics research. A human synovial joint is a complex tribological system which is able to minimize friction and wear of the articulated cartilage surfaces, thanks to the mix of particular natural lubrication regimes which are established within the synovial membrane. Mainly due to the aging process, the cartilage covering the bones linked by the synovial joint tends to deteriorate, causing direct bone-to-bone contact and joint wear: the synovial joint diseases are responsible for loss of mobility, pain and tissues’ inflammation. The replacement of the worn natural joint with a prosthesis is actually needed, so a detailed tribological analysis of the artificial implant has to be conducted in order to predict the artificial joint wear and to maximize its duration. While the in vitro approach is obviously reliable because it is based on experimental tests, the in-silico analysis is surely cheaper but it needs a deep and detailed mathematical and physical description of the analysed phenomena. In this chapter the works produced by the authors are presented and the biotribology of the total hip replacements is discussed. In particular, once the numerical modelling of the synovial mixed elasto-hydrodynamic lubrication referred to the artificial hip joint is analysed, the influence of the joint configuration in terms of load and relative motion on its tribological state is introduced as the output of a pure mechanical problem associated with the human musculoskeletal system. The kinematics of the musculoskeletal system plays a key role within the joint tribological analysis, because it defines its dynamical behaviour and, in dependence on the muscular actions, generates the joint reactions which couple the articulations and affect its lubrication/contact state. A multibody model of the human lower limb is shown and the inverse dynamics of the system subjected to a particular kinematics is described and solved. The aim of this chapter is to propose an algorithm characterised by a direct workflow which starts from the musculoskeletal kinematics and, coupling the multibody and the lubrication models, leads to the calculation of interesting tribological quantities. The proposed approach can be established as a powerful in silico tool adaptable to a particular subject and to several kinematics, in order to analyse the artificial joint tribology of customized implants.