Time dependent flow of biolubricant and suspended particles behavior within total hip replacement

Sammanfattning: Total hip replacement (THR) has been one of the most successful surgeries in the 21st century. Ultra-high-molecular-weight-polyethylene (UHMWPE) shows favorable mechanical and tribological properties when used as a bearing surface material in THR. However, produced UHMWPE wear particles challenge increasing the THR lifetimes. Bone loss (osteolysis) initiated by these wear particles is a major cause of total joint arthroplasty failure in both hip and knee prosthesis.In addition to improving the wear resistance of bearing surfaces to reduce wear, wear debris distribution mechanisms within the joint gap must also be thoroughly investigated. These particles distribute within lubricant and across the implant gap. Synovial fluid (SF) lubricates natural joints which is a viscoelastic non-Newtonian shear thinning fluid. The non-Newtonian behavior of SF is attributed to its hyaluronic acid (HA) content which is a linear biopolymer. The distribution patterns of wear particles within total joint replacement are affected by the special rheological behaviors of the SF, geometrical parameters, particle size and shape distribution and particle-fluid interactions. Therefore, understanding wear particles distribution pattern is pivotal to understand the mechanism and eventually minimizing third-body wear of the UHMWPE acetabular liner in THR.According to fluid mechanics forces, the size and density of wear particles suggests that wear particles follow lubricant movements. However, over a matter of hours, such particles show specific behaviors within viscoelastic fluid (not visible in Newtonian fluid) such as particle migration and string formation along the flow direction.The main aim of this project was to develop and validate an experimental method for assessing characteristics of HA and artificial SF solutions and behaviors of wear particles in a viscoelastic fluid flow. The effects of different parameters such as HA concentration, protein content, fluid flow types (steady, unsteady, etc.) and gap shapes on fluid behavior were quantified to fully understand such mechanisms.In this project, micro particle image velocimetry (micro-PIV) was applied as the quantitative flow visualization method. Pin-on-disk tribo-measurement was performed as a complimentary study to investigate the tribological behaviors of a UHMWPE pin rotating against a cobalt chromium molybdenum (CoCrMo) disk in the presence of HA solutions of various concentrations. This study was conducted to understand the effects of dynamic loading on lubricant performance relative to those of static loading.The results showed that under oscillatory flow conditions, strain levels, rates, and distributions are important parameters that affect the flow behaviors of HA solutions. Particle migration and alignment were affected by channel sizes, HA concentrations, flow types and the elastic instability of the solution.The tribological study results suggested that in the presence of HA, sinusoidal dynamic loading does not affect the frictional behaviors of UHMWPE moving against CoCrMo in comparison with static loading.