Numerical solution of the theoretical model of knee prosthesis for the determination of appropriate materials parameters

MARCELO E. BERLI; DIEGO M. CAMPANA; SEBASTIÁN UBAL; JOSÉ DI PAOLO

Rosario, Argentina

Congreso; 1º Taller de Órganos Artificiales, Biomateriales e Ingeniería de Tejidos (BIOMAT 2009); 2009

Knee prostheses are currently the only solution for totally worn out knees. One of the major problems is the aseptic loosening of the implant, resulting from wear of ultra high molecular weigh polyethylene (UHMWPE). Science and technology are focused on the improvement of the mechanical response of UHMWPE and, at the same time, the development of new cartilage replacement materials. The aim of this work is to simulate the lubrication mechanism of both a knee prosthesis and a healthy knee in order to investigate the influence of the mechanical variables (pressure, film thickness and friction coefficient) on wear. To this end, we built a model of the lubrication between the femoral head and the tibial plateau, resulting in a set of nonlinear equations solved by means of a numerically technique based on the finite element method. The model takes into account the non-Newtonian characteristic of the synovial fluid, the concentration of the hyaluronic acid, as well as the visco-elasticity and exudation factor (porosity) of a hypothetical new replacement material. The numerical results show that UHMWPE has a very high elastic modulus to provide appropriated lubrication to the joint, leading to very large pressure values and very thin film thicknesses in the lubrication channel. However, a replacement material displaying visco-elastic characteristics showed thinner film thicknesses (of the order of the superficial roughness) even for a soft material, a very undesirable feature from the standpoint of wear. In this sense, the high rigidity of UHMWPE contributes to decrease the friction coefficient compared to softer materials. We also identify the mechanical features of a healthy cartilage responsible for the proper lubrication conditions and therefore low wear of natural joints, which should be employed as guidelines in the design of new engineered tissues.