Mechanical and tribological investigation on natural and artificial human joints

Synovial joints of the human lower limb, e.g. the knee and the hip, are characterized by a wide mobility and can support very high loads, up to several times the body weight. Their functionality is fundamental to guarantee the main daily activities, such as the walking, and, as a consequence, when affected by pathological conditions, can strongly limit the normal life. Unfortunately, often these joints can lose their functionality and need to be replaced with joint implants by means of an orthopaedic surgery called arthroplasty. The osteoarthritis (OA), which consists in a damage of the articular cartilage (AC), is the most frequent cause of disability affecting the elderly population and is also the primary cause of joint replacements. A type of OA, called secondary OA, has generated much interest in the engineering world since it is thought to be mechanically induced, due to abnormal and excessive stresses and strains occurring in the tissue. During the stage period at the Imperial College London (London, UK), the mechanical behaviour of the AC has been investigated with the final aim of evaluating critical conditions likely causing cartilage damage [1, 2]. Since the knee meniscectomy is recognized as a possible cause of OA, the mechanical response of the AC in a patient specific model of a meniscectomised knee has been studied. Advanced material models of the AC have been implemented in a realistic 3D geometry for the first time in the literature, showing the importance to model the AC as a biphasic tissue in order to accurately predict the its mechanical response and the onset of AC damages. As mentioned above, in case of osteoarthritic joints, typically affected by chronic pain and reduced mobility, the arthroplasty is considered necessary. Many researchers have been investigating the optimization of implant design, in order to guarantee the same biomechanics of the natural joints, and to avoid the wear of the articulating surfaces, which is considered one of the main causes of implant failures. In the last few years, there has been an increasing amount of literature on the biotribology of hip implants, whose structure, geometry and kinematics, more simple in respect with the knee replacements, make easier both theoretical and experimental studies. On the basis of an in-depth review of the lubrication models in the literature, the elasto-hydrodynamic lubrication (EHL) model of hard-on-hard hip implants has been developed to predict and compare in-vivo performances of hip implants with different geometry and materials: the results obtained can explain the actual clinical trends and provide a sensitivity analysis of lubrication to some design parameters. Also some preliminary theoretical and experimental studies) on the wear of hip implants have been carried out in collaboration with the Istituti Ortopedici Rizzoli (Bo, IT). In the future the combination of wear and lubrication models will be addressed.