Joint contact modelling of articular cartilage in synovial joints

Hip joint is one of the important load bearing joints and has been extensively studied to investigate contact mechanics and tribology. It has known to experience
high contact forces and stresses. However, cartilage shows remarkable lubricating and wear properties, and survives the lifetime of a person. Biphasic lubrication based on the principle of fluid load support has provided an explanation for this. However, when, the cartilage fails the part or whole of the joint needs to be replaced and hemiarthroplasty is one such remedy.

Three-dimensional finite element models with elastic/hyperelastic cartilage have been used to investigate contact mechanics of the hip joint. However, to
understand the role of interstitial fluid in contact mechanics and tribology, cartilage has to be modelled as biphasic material. Interventions such as hemiarthroplasty
may alter this phenomenon and hence it is also important to know the extent of this effect. This study was thus an attempt to address these issues.

An algorithm developed earlier for 2-D problems was refined, adapted and tested for 3-D problems to detect nodes in contact to impose surface fluid flow conditions. This was then used in natural hip joint where fluid load support was found to be very high (~94%). Three-dimensional hemiarthroplasty was then experimentally verified using porcine hips. The methodology was then used to investigate the effect of clearance in hemiarthroplasty which confirmed the earlier findings that undersizing of the femoral head increases both contact and shear stresses probably leading to cartilage erosion. The investigation of the activities of
daily living showed lower contact stresses when compared to the outcomes of clinical studies and depended not only on the magnitude of the load but also on their locations. In all the models the total fluid load support was very high and was between ~90% which supported the biphasic lubrication hypothesis.