Analysis of wear and friction of total knee replacements part II: Friction and lubrication as a function of wear

Using a friction measurement system designed for such a purpose the frictional characteristics of a total knee replacement system have been assessed as a function of in vitro wear following wear simulation as described in Part I. The Stribeck analysis has been used to evaluate the data and to indicate the mode of lubrication. Using carboxymethyl cellulose (CMC) solutions as the lubricant it was observed that the UHMWPE total knee replacement (TKR) sets tested exhibited friction factors that fluctuated between 0.01 and 0.05. There was no relationship between the Stribeck plots of the TKRs at the different stages of wear testing. Overall, Stribeck analysis of the TKR sets appeared to indicate a mixed lubrication regime with decreasing friction factors as the Sommerfeld number increased. However, the analysis of Pearsonʹs correlation confirmed that this was a statistically weak relationship. It is proposed that the observed variability in the frictional measurements can be accounted for by the joints operating in a mixed lubrication regime where the friction depends on asperity contacts between the articulating counterfaces. The friction factors of the joints depend on the type and content of the lubricant used. Protein containing lubricants caused an increase in friction relative to the CMC solutions and it is thought that this arises from interfacial contact of proteins adsorbed on the articulating surfaces. However, increasing protein content from 1.67 to 5.02 g/dl did not result in a significant increase in friction while the addition of a hyaluronic acid solution resulted in a decrease in the friction factor.