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Submission Abstract (max 120 words)In vitro buffers are frequently used to formulate model synovial fluids to investigate the role of individual constituents in synovial joint lubrication. This work examines how buffer choice affects protein film formation in static and rolling conditions. Solution pH dominates both the protein adsorption kinetics and the formation of tribofilms in static and rolling conditions respectively. Under static conditions, equilibrium adsorbed protein films from all buffers tested have similar properties although three distinct modes of adsorption, governed by the pH of the buffer, are observed. Films formed under rolling conditions are also pH dependent, with large irregular deposits formed in solutions with pH 7.4 or lower, The adsorption properties of proteins under static conditions only partially govern the lubrication properties of proteins. AbstractSynovial fluid contains proteins which can participate in surface film formation and the lubrication of artificial joints. In laboratory experiments, model synovial fluids with a range of buffers are frequently used by various investigators. Buffer compositions with different pH, ionic strength and buffer chemistry may affect protein adsorption, and hence film formation. To clarify the role of buffers in protein film formations, the static adsorption of BSA in a selection of buffers is studied using a quartz crystal microbalance. Three distinct modes of adsorption are observed, but on reaching equilibrium, the adsorbed BSA layers have similar viscoelastic properties in all buffer solutions. The static adsorption results are compared with lubricants film formed in model rolling contacts. Films formed under rolling conditions are pH dependent. Thin, uniform films consistent with adsorbed films, and nonuniform films, with large irregular deposits are formed in high and low pH solutions respectively. The results show that the pH of the solution dominates both the adsorption kinetics of proteins under static conditions and the formation of tribofilms in rolling conditions. However, the adsorption properties of proteins under static conditions only partially govern the lubrication property of protein solutions.

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A B S T R A C TA new test was developed to measure friction and wear of hip implant materials under reciprocating sliding conditions. The method requires a very small amount of lubricant (< 3 ml) which allows testing of human synovial fluid. Friction and wear of Cobalt Chromium Molybdenum (CoCrMo) material pairs were measured for a range of model and human synovial fluid samples. The initial development of the test assessed the effect of fluid volume and bovine calf serum (BCS) concentration on friction and wear. In a second series of tests human synovial fluid (HSF) was used. The wear scar size (depth and volume) on the disc was dependent on protein content and reduced significantly for increasing BCS concentration. The results showed that fluid volumes of < 1.5 ml were affected by evaporative loss effectively increasing the protein concentration resulting in anomalously lower wear. At the end of the test thick deposits were observed in and around the wear scars on the disc and ball; these were analysed by Infrared Reflection-Absorption Spectroscopy. The deposits were composed primarily of denatured proteins and similar IR spectra were obtained from the BCS and HSF tests. The analysis confirmed the importance of SF proteins in determining wear of CoCrMo couples.

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Zirconia in Zirconia toughened alumina ceramic hip replacements exists in an unstable state and can transform in response to stress giving the material improved fracture toughness. Phase transformation also occurs under hydrothermal conditions such as exist in vivo. To predict the hydrothermal aging that will occur in vivo accelerated aging procedures have been used, but validation of these models requires the study of retrieved hip joints. Here 26 retrievals are analysed to determine the degree of phase transformation in vivo. These were compared with virgin heads, heads that had undergone the accelerated aging process and heads wear tested to 5 million cycles in a hip simulator. Monoclinic content and surface roughness were measured using Raman spectroscopy and white light interferometry respectively. The monoclinic content for retrieved heads was 28.5% ± 7.8, greater than twice that in virgin, aged, or wear tested heads and did not have a significant correlation with time, contrary to the predictions of the hydrothermal aging model. The surface roughness for retrieved heads in the unworn area was not significantly different to that in virgin, aged, or unworn areas of wear tested heads. However in worn areas of the retrieved heads, the surface roughness was higher than observed in wear simulator testing. These results indicate that current testing methodologies do not fully capture the operational conditions of the material and the real performance of future new materials may not be adequately predicted by current pre‐clinical testing methods. © 2017 The Authors. Journal of Orthopaedic Research Published by Wiley Periodicals, Inc. on behalf of Orthopaedic Research Society 35:2781–2789, 2017.

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