154 results found
Stevenson H, Jaggard M, Akhbari P, et al., 2019, The role of denatured synovial fluid proteins in the lubrication of artificial joints, Biotribology, Vol: 17, Pages: 49-63, ISSN: 2352-5738
CoCrMo ball-on-flat wear tests were carried out with 25 wt% bovine calf serum (25BCS) and human synovial fluid (HSF) to investigate artificial joint lubricating mechanisms. Post-test the wear scar on the disc was measured and surface deposits in and around the rubbed region were analysed by Micro InfraRed Reflection Absorption Spectroscopy (Micro-IRRAS). In most tests the HSF samples gave higher wear than the 25BCS solution; in some cases, up to 77%. After rinsing a similar pattern of surface deposits was observed in and around the wear scar for both the model and HSF. Micro-IRRAS showed the deposits were primarily denatured proteins with an increased β-sheet content. In some cases, trans-alkyl chain/carbonyl components were also present and these were assigned to lipids. Thioflavin T fluorescent imaging also indicated aggregated non-native β-sheet fibrils were present in the deposits and their presence was associated with lower wear. The formation of insoluble, denatured protein films is thought to be the primary lubrication mechanism contributing to surface protection during rubbing. From this and earlier work we suggest inlet shear induces denaturing of proteins resulting in the formation of non-native β-sheet aggregates. This material is entrained into the contact region where it forms the lubricating film. Patient synovial fluid chemistry appears to influence wear, at least in the bench test, and thus could contributes to increased risk of failure, or success, with metal-metal hips. Finally using 25BCS as a reference screening fluid gives an overly optimistic view of wear in these systems.
Porte E, Cann P, Masen M, 2019, Fluid load support does not explain tribological performance of PVA hydrogels, Journal of the Mechanical Behavior of Biomedical Materials, Vol: 90, Pages: 284-294, ISSN: 1751-6161
© 2018 Elsevier Ltd The application of hydrogels as articular cartilage (AC) repair or replacement materials is limited by poor tribological behaviour, as it does not match that of native AC. In cartilage, the pressurisation of the interstitial fluid is thought to be crucial for the low friction as the load is shared between the solid and liquid phase of the material. This fluid load support theory is also often applied to hydrogels. However, this theory has not been validated as no experimental evidence directly relates the pressurisation of the interstitial fluid to the frictional response of hydrogels. This lack of understanding about the governing tribological mechanisms in hydrogels limits their optimised design. Therefore, this paper aims to provide a direct measure for fluid load support in hydrogels under physiologically relevant sliding conditions. A photoelastic method was developed to simultaneously measure the load on the solid phase of the hydrogel and its friction coefficient and thus directly relate friction and fluid load support. The results showed a clear distinction in frictional behaviour between the different test conditions, but results from photoelastic images and stress-relaxation experiments indicated that fluid load support is an unlikely explanation for the frictional response of the hydrogels. A more appropriate explanation, we hypothesized, is a non-replenished lubricant mechanism. This work has important implications for the tribology of cartilage and hydrogels as it shows that the existing theories do not adequately describe the tribological behaviour of hydrogels. The developed insights can be used to optimise the tribological performance of hydrogels as articular cartilage implants.
Stevenson H, Parkes M, Austin L, et al., 2018, The development of a small-scale wear test for CoCrMo specimens with human synovial fluid, Biotribology, Vol: 14, Pages: 1-10, ISSN: 2352-5738
A 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.
Masen M, Cann PME, 2018, Friction measurements with molten chocolate, Tribology Letters, Vol: 66, ISSN: 1023-8883
A novel test is reported which allows the measurement of the friction of molten chocolate in a model tongue–palate rubbing contact. Friction was measured over a rubbing period of 150 s for a range of commercial samples with different cocoa content (85–5% w/w). Most of the friction curves had a characteristic pattern: initially a rapid increase occurs as the high-viscosity chocolate melt is sheared in the contact region followed by friction drop as the film breaks down. The exceptions were the very high (85%) and very low (~ 5%) cocoa content samples which gave fairly constant friction traces over the test time. Differences were observed in the initial maximum and final friction coefficients depending on chocolate composition. Generally, the initial maximum friction increased with increasing cocoa content. At the end of the test, the rubbed films on the lower slide were examined by optical microscopy and infrared micro-reflection spectroscopy. In the rubbed track, the chocolate structure was severely degraded and predominately composed of lipid droplets, which was confirmed by the IR spectra. The new test provides a method to distinguish between the friction behaviour of different chocolate formulations in a rubbing low-pressure contact. It also allows us to identify changes in the degraded chocolate film that can be linked to the friction profile. Further development of the test method is required to improve simulation of the tongue–palate contact including the effect of saliva and this will be the next stage of the research.
Milner P, Parkes M, Puetzer J, et al., 2017, A Low Friction, Biphasic and Boundary Lubricating Hydrogel for Cartilage Replacement, Acta Biomaterialia, Vol: 65, Pages: 102-111, ISSN: 1742-7061
Partial joint repair is a surgical procedure where an artificial material is used to replace localised chondral damage. These artificial bearing surfaces must articulate against cartilage, but current materials do not replicate both the biphasic and boundary lubrication mechanisms of cartilage. A research challenge therefore exists to provide a material that mimics both boundary and biphasic lubrication mechanisms of cartilage.In this work a polymeric network of a biomimetic boundary lubricant, poly(2-methacryloyloxyethyl phosphorylcholine) (PMPC), was incorporated into an ultra-tough double network (DN) biphasic (water phase + polymer phase) gel, to form a PMPC triple network (PMPC TN) hydrogel with boundary and biphasic lubrication capability. The presence of this third network of MPC was confirmed using ATR-FTIR. The PMPC TN hydrogel had a yield stress of 26 MPa, which is an order of magnitude higher than the peak stresses found in the native human knee. A preliminary pin on plate tribology study was performed where both the DN and PMPC TN hydrogels experienced a reduction in friction with increasing sliding speed which is consistent with biphasic lubrication. In the physiological sliding speed range, the PMPC TN hydrogel halved the friction compared to the DN hydrogel indicating the boundary lubricating PMPC network was working.A biocompatible, tough, strong and chondral lubrication imitating PMPC TN hydrogel was synthesised in this work. By complementing the biphasic and boundary lubrication mechanisms of cartilage, PMPC TN hydrogel could reduce the reported incidence of chondral damage opposite partial joint repair implants, and therefore increase the clinical efficacy of partial joint repair.Statement of SignificanceThis paper presents the synthesis, characterisation and preliminary tribological testing of a new biomaterial that aims to recreate the primary chondral lubrication mechanisms: boundary and biphasic lubrication. This work has demonstrated that the
De Laurentis N, Cann P, Lugt P, et al., 2017, The Influence of Base Oil Properties on the Friction Behaviour of Lithium Greases in Rolling/Sliding Concentrated Contacts, Tribology Letters, Vol: 65, ISSN: 1023-8883
This study investigates the influence of base oil type and viscosity on the frictional behaviour of lithium-thickened bearing greases. A series of model lithium greases were manufactured by systematically varying viscosity and type of base oil, so that the influence of a single base oil property could be studied in isolation. In addition, selected greases were blended with oleic acid, with the purpose of evaluating its effectiveness in further reducing grease friction. Friction coefficient and film thickness were measured in laboratory ball-on-disc tribometers over a range of speeds and temperatures. For a specific oil type, the influence of base oil viscosity on friction was found to be closely related to its effect on film thickness: greases formulated with PAO oils covering a wide range of viscosities gave very similar friction at the same nominal film thickness. For a given base oil viscosity, base oil type was found to have a strong influence on grease friction under all test conditions. PAO-based greases generally produced lower friction than mineral- and ester-based greases. Addition of oleic acid to the test greases did not significantly affect friction within the range of test conditions employed in this study. The results provide new insight into the frictional behaviour of greases, which may be used to help inform new low-friction grease formulations for rolling bearing applications.
Parkes M, Cann P, Jeffers J, 2017, Real-time observation of fluid flows in tissue during stress relaxation using Raman spectroscopy, Journal of Biomechanics, Vol: 60, Pages: 261-265, ISSN: 1873-2380
This paper outlines a technique to measure fluid levels in articular cartilage tissue during an unconfined stress relaxation test. A time series of Raman spectrum were recorded during relaxation and the changes in the specific Raman spectral bands assigned to water and protein were monitored to determine the fluid content of the tissue. After 1000 s unconfined compression the fluid content of the tissue is reduced by an average of 3.9% ± 1.7%. The reduction in fluid content during compression varies between samples but does not significantly increase with increasing strain. Further development of this technique will allow mapping of fluid distribution and flows during dynamic testing making it a powerful tool to understand the role of interstitial fluid in the functional performance of cartilage.
Parkes M, Sayer K, Goldhofer M, et al., 2017, Zirconia phase transformation in retrieved, wear simulated and artificially aged ceramic femoral heads, Journal of Orthopaedic Research, Vol: 35, Pages: 2781-2789, ISSN: 1554-527X
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. This article is protected by copyright. All rights reserved
Tsui S, Tandy J, Myant C, et al., 2016, Friction measurements with yoghurt in a simulated tongue-palate contact, Biotribology, Vol: 8, Pages: 1-11
© 2016 The perception of many food attributes is related to mechanical stimulation and friction experienced in the tongue-palate contact during mastication. Friction in the tongue-palate is determined by the changing film properties (composition, component distribution, thickness) in the conjunction. We suggest this evolution is essentially determined by tongue-palate film loss rather than shear flow entrainment which predominates in conventional bearing lubrication. The paper reports friction measurements in a simulated tongue-palate contact for a range of high and low fat dairy foods. A reciprocating, sliding contact with restricted stroke length ( < contact width) was used; under these conditions there is negligible shear-entrainment of fluid from outside the contact area. The tongue-palate contact was simulated by a PDMS ball and glass surface. The effect of hydrophobic and hydrophilic surfaces on friction was investigated for different fat contents (0, 4.2, 9.5% wt fat). Friction was measured over 60 s of rubbing. Significant differences were observed in the friction change with time for different fat contents (μ 9.5 < μ 4.2 < μ 0 wt%) and for different surface energy conditions (μ hydrophilic < μ hydrophobic). Post-test visualisation of the rubbed films showed that low friction coefficient was associated with the formation of a thin oil film on deposited particulate solids.
Huang L, Guo D, Cann PM, et al., 2016, Thermal Oxidation Mechanism of Polyalphaolefin Greases with Lithium Soap and Diurea Thickeners: Effects of the Thickener, Tribology Transactions, ISSN: 1040-2004
In this work, lithium and diurea greases formulated by poly-alpha-olefin were aged up to 1,200 h in an oven at 120°C and periodically taken out for testing. Scanning electron microscopy (SEM) and infrared (IR) spectra proved that both physical and chemical degradation occurred during the thermal aging process, such as a decrease in apparent viscosity, thickener destruction, and change in chemical species. Diurea grease showed much better anti-oxidation performance during thermal aging than lithium grease. A dual effect of thermal aging on the grease lubricity was observed and analyzed. Results showed that early oxidation might reduce grease lubricity due to the formation of ketones and aldehydes, and the decrease in grease viscosity and oil bleeding due to thickener destruction would contribute to better replenishment.
De Laurentis N, Kadiric A, Lugt P, et al., 2016, The influence of bearing grease composition on friction in rolling/sliding concentrated contacts, Tribology International, Vol: 94, Pages: 624-632, ISSN: 0301-679X
This paper presents new results examining the relationship between bearing grease composition and rolling-sliding friction in lubricated contacts. Friction coefficient and lubricating film thickness of a series of commercially available bearing greases and their bled oils were measured in laboratory tribometers. Test greases were selected to cover a wide spectrum of thickener and base oil types, and base oil viscosities. The trends in measured friction coefficients were analysed in relation to grease composition in an attempt to establish the relative influence of individual grease components on friction. Two distinct operating regions with markedly different friction behaviour are identified for each grease. At relatively high speeds the greases behave approximately as their bled/base oils, while in the low speed region the frictional response is very dependent on their thickener type and properties of the lubricating film. Low viscosity, synthetic base oil seems to offer efficiency advantages in the high speed region regardless of thickener used, while the choice of thickener type is significant under low speed conditions.
Parkes M, Myant C, Cann PM, et al., 2015, Synovial fluid lubrication: The effect of protein interactions on adsorbed and lubricating films, Biotribology, Vol: 1-2, Pages: 51-60, ISSN: 2352-5738
© 2015 Elsevier Ltd. All rights reserved. Synovial fluid lubrication is dependent on protective protein films that form between joint surfaces. Under static conditions surface film formation occurs through adsorption, while under dynamic conditions protein aggregation under shear and load becomes the dominant mechanism. This work examines how the protein content of six model synovial fluids affects film formation under static and rolling conditions and if the changes in properties can be correlated. With an increase in the statically adsorbed mass and the rate of adsorption the film thickness under rolling increased. These increases did not correlate with the total protein content of the fluid, but were dependent on the type of protein. An increase in pH reduced the adsorbed mass, rate of adsorption and film thickness, but was of secondary importance to the type of protein. The rolling film thickness was also correlated with the viscoelastic properties of the films formed under static conditions. In this case thinner rolling films corresponded to the more hydrated, viscoelastic adsorbed films. The strong correlations found between the properties of the adsorbed films and those formed under rolling indicate that the same protein-protein and protein-surface interactions may govern both mechanisms of film formation despite the differences in the film structures.
Parkes M, Myant C, Dini D, et al., 2014, Tribology-optimised silk protein hydrogels for articular cartilage repair, Tribology International, Pages: ---, ISSN: 0301-679X
Myant C, Cann P, 2014, On the matter of synovial fluid lubrication: Implications for Metal-on-Metal hip tribology, Journal of the Mechanical Behavior of Biomedical Materials, Vol: 34, Pages: 338-348, ISSN: 1751-6161
Artificial articular joints present an interesting, and difficult, tribological problem. These bearing contacts undergo complex transient loading and multi axes kinematic cycles, over extremely long periods of time (>10 years). Despite extensive research, wear of the bearing surfaces, particularly metal–metal hips, remains a major problem. Comparatively little is known about the prevailing lubrication mechanism in artificial joints which is a serious gap in our knowledge as this determines film formation and hence wear. In this paper we review the accepted lubrication models for artificial hips and present a new concept to explain film formation with synovial fluid. This model, recently proposed by the authors, suggests that interfacial film formation is determined by rheological changes local to the contact and is driven by aggregation of synovial fluid proteins. The implications of this new mechanism for the tribological performance of new implant designs and the effect of patient synovial fluid properties are discussed.
Myant CW, Cann P, 2014, The effect of transient conditions on synovial fluid protein aggregation lubrication, Journal of the Mechanical Behavior of Biomedical Materials, Vol: 34, Pages: 349-357, ISSN: 1751-6161
Little is known about the prevailing lubrication mechanisms in artificial articular joints and the way in which these mechanisms determine implant performance. The authors propose that interfacial film formation is determined by rheological changes local to the contact and is driven by aggregation of synovial fluid proteins within the contact inlet region. A direct relationship between contact film thickness and size of the protein aggregation within the inlet region has been observed.In this paper the latest experimental observations of the protein aggregation mechanism are presented for conditions which more closely mimic joint kinematics and loading. Lubricant films were measured for a series of bovine calf serum solutions for CoCrMo femoral component sliding against a glass disc. An optical interferometric apparatus was employed to study the effects of transient motion on lubricant film formation. Central film thickness was measured as a function of time for a series of transient entrainment conditions; start-up motion, steady-state and non-steady-state uni-directional sliding, and bi-directional sliding. The size of the inlet aggregations was found to be dependent upon the type of transient condition. Thick protective protein films were observed to build up within the main contact region for all uni-directional tests. In contrast the inlet aggregation was not observed for bi-directional tests. Contact film thickness and wear was found to be directly proportional to the presence of the inlet protein phase. The inlet phase and contact films were found to be fragile when disrupted by surface scratches or subjected to reversal of the sliding direction.
Parkes M, Myant C, Cann PM, et al., 2014, The effect of buffer solution choice on protein adsorption and lubrication, Tribology International, Vol: 72, Pages: 108-117, ISSN: 0301-679X
Myant CW, fowell M, cann P, 2013, The effect of transient motion on Isoviscous-EHL films in compliant, point, contacts, Tribology International, Vol: 72, Pages: 98-107, ISSN: 1879-2464
Laser induced fluorescence was employed to measure lubricant film thickness in a compliant, point, contact during transient motion. Two types of transient sliding motion were investigated: start-up and sudden halting. The effects of acceleration rate and sliding speed on film formation and breakdown were studied.A clear relationship between start-up acceleration and the period of the film formation phase was observed. During sudden halting motion entrapment of fluid occurred in the centre of the contact. This trapped fluid was squeezed out of the contact over several seconds. The size of this entrapment was dependent on the initial sliding speed. The findings are compared to similar results for hard, point, contacts and the implications discussed.
Accardi MA, McCullen SD, Callanan A, et al., 2013, Effects of fiber orientation on the frictional properties and damage of regenerative articular cartilage surfaces, Tissue Engineering: Parts A, B, and C, Vol: 19, Pages: 2300-2310, ISSN: 1937-3368
Articular cartilage provides a low-friction, wear-resistant surface for diarthrodial joints. Due to overloading and overuse, articular cartilage is known to undergo significant wear and degeneration potentially resulting in osteoarthritis (OA). Regenerative medicine strategies offer a promising solution for the treatment of articular cartilage defects and potentially localized early OA. Such strategies rely on the development of materials to restore some aspects of cartilage. In this study, microfibrous poly(ɛ-caprolactone) scaffolds of varying fiber orientations (random and aligned) were cultured with bovine chondrocytes for 4 weeks in vitro, and the mechanical and frictional properties were evaluated. Mechanical properties were quantified using unconfined compression and tensile testing techniques. Frictional properties were investigated at physiological compressive strains occurring in native articular cartilage. Scaffolds were sheared along the fiber direction, perpendicular to the fiber direction and in random orientation. The evolution of damage as a result of shear was evaluated via white light interferometry and scanning electron microscopy. As expected, the fiber orientation strongly affected the tensile properties as well as the compressive modulus of the scaffolds. Fiber orientation did not significantly affect the equilibrium frictional coefficient, but it was, however, a key factor in dictating the evolution of surface damage on the surface. Scaffolds shear tested perpendicular to the fiber orientation displayed the highest surface damage. Our results suggest that the fiber orientation of the scaffold implanted in the joint could strongly affect its resistance to damage due to shear. Scaffold fiber orientation should thus be carefully considered when using microfibrous scaffolds.
Myant C, Cann P, 2013, In contact observation of model synovial fluid lubricating mechanisms, 1st International Conference on Biotribology (ICoBT), Publisher: ELSEVIER SCI LTD, Pages: 97-104, ISSN: 0301-679X
Hart AJ, Muirhead-Allwood S, Porter M, et al., 2013, Which Factors Determine the Wear Rate of Large-Diameter Metal-on-Metal Hip Replacements?, JOURNAL OF BONE AND JOINT SURGERY-AMERICAN VOLUME, Vol: 95A, Pages: 678-685, ISSN: 0021-9355
Myant C, Cann P, 2013, Lubrication of artificial articular joints, Pages: 132-134
Ingram M, Underwood R, Denyer P, et al., 2013, The development of a laboratory screening method to optimize lubrication maintenance of high voltage equipment, NLGI Spokesman, Vol: 76, Pages: 9-21, ISSN: 0027-6782
Imperial College and National Grid Electricity Transmission carried out a study to evaluate lubrication maintenance of electrical switchgear used throughout the network. A discussion on this project covers the switchgear components and lubrication requirements; identification of lubricant degradation mechanisms; development of pertinent screening tests; survey of current lubricants used; and recommendations for the future.
Underwood RJ, Kocagoz SB, Smith R, et al., 2013, A protocol to assess the wear of head/neck taper junctions in large head metal-on-metal (LHMoM) hips, ASTM Special Technical Publication, Vol: 1560 STP, Pages: 209-234, ISSN: 0066-0558
Researchers have hypothesized that the increased revision rate of LHMoM (Large Head Metal-on-Metal) hips compared to MoM hip resurfacings may be attributed to corrosion and wear at the head neck taper junction. Studies have reported visual evidence of fretting and corrosion at the taper junction, but no method has been described in the literature to quantify the amount of material lost from the taper junction. This paper describes a measurement protocol using a Taylor Hobson Talyrond Roundness instrument that allows the simultaneous measurement of surface form (wear) and surface topography (roughness). The methodology allows the measurement of the taper angle, geometry of worn region (depth, length), 3D surface maps and surface topography of the head conical taper. The accurate quantification of the taper geometry, wear and topography is essential to the understanding of the in vivo wear and corrosion mechanisms of taper junctions in LHMoM hips. Copyright © 2013 by ASTM International.
Underwood RJ, Fowell M, Sayles R, et al., 2013, The development of a standard method for assessing wear of explanted metal-on-metal hip joints, ASTM Special Technical Publication, Vol: 1560 STP, Pages: 130-145, ISSN: 0066-0558
The concerns surrounding the current generation of metal-on- metal (MoM) hips and the regulatory requirements regarding the reporting of revised MoM components highlight the need for a standardised method for measuring the wear of explanted MoM hip components. This paper reviews the published measurement and analysis protocols used in studies of explanted MoM hips. Because of differences in the measurement and analysis techniques, it is not reliable to directly compare measurements between studies. This paper considers the requirements for a standardised measurement protocol and forms the rationale for a proposed international standard. Copyright © 2013 by ASTM International.
Myant CWI, Cann PME, 2013, In contact observation of model synovial fluid lubricating mechanisms, Tribology International, Vol: 63, Pages: 97-104, ISSN: 0301-679X
This paper examines the fundamental mechanisms of synovial fluid lubrication in artificial joints. Film thickness measurements were made for bovine serum solutions in a model test device. In contact imaging was also carried out to aid interpretation of these results. The results indicated that two types of film are formed; a boundary layer of adsorbed protein molecules, which are augmented by a high-viscosity fluid film generated by hydrodynamic effects. The high-viscosity film is due to inlet aggregation of protein molecules forming a gel which is entrained into the contact preferentially at low speeds. As the speed increases this gel appears to shear thin, giving much lower lubricant film thickness. Results suggest that protein-containing fluids do not obey classical Newtonian EHL models. © 2012 Elsevier Ltd. All rights reserved.
Myant CW, Fan T, Underwood R, et al., 2012, Synovial Fluid Lubrication of Artificial Joints: Protein Film Formation and Composition, Faraday Discussions, ISSN: 1364-5498
Underwood RJ, Zografos A, Sayles RS, et al., 2012, Edge loading in metal-on-metal hips: low clearance is a new risk factor, PROCEEDINGS OF THE INSTITUTION OF MECHANICAL ENGINEERS PART H-JOURNAL OF ENGINEERING IN MEDICINE, Vol: 226, Pages: 217-226, ISSN: 0954-4119
Fan J, Myant C, Underwood R, et al., 2012, Synovial fluid lubrication of artificial joints: protein film formation and composition, FARADAY DISCUSSIONS, Vol: 156, Pages: 69-85, ISSN: 1359-6640
Bills PJ, Racasan R, Underwood RJ, et al., 2012, Volumetric wear assessment of retrieved metal-on-metal hip prostheses and the impact of measurement uncertainty, Wear, Pages: 212-219
Myant CW, underwood R, fan J, et al., 2011, Lubrication of metal-on-metal hip joints: The effect of proteincontent and load on film formation and wear, Journal of the Mechanical Behavior of Biomedical Materials, Vol: 6, Pages: 30-40, ISSN: 1751-6161
Lubricant films were measured for a series of bovine serum and protein containing (albumin, globulin) saline solutions for CoCrMo femoral component sliding against a glass disc. Central film thickness was measured by optical interferometry as a function of time (constant mean speed: 0 and 10 mm/s) and variable mean speed (0–50 mm/s). The effect of load (5–20 N) on film thickness was also studied. The development of the wear scar on the CoCrMo surface was monitored by measuring the width of the contact zone during the film thickness tests. The results showed film thickness increased with time for both the static and sliding tests. Films formed in the static, loaded test were typically in the range of 3–40 nm. The globulin containing solutions formed the thickest films. In the sliding tests a wear scar rapidly formed on the implant component for the bovine serum and albumin fluids, negligible wear was observed for the globulin solutions. Film thickness increased with sliding time for all test solutions and was much greater than predicted by isoviscous EHL models. The film increase was found to correlate with increasing wear scar size and thus decreasing contact pressure. A new lubricating mechanism is proposed whereby during sliding the fluid undergoes bulk phase separation rheology, so that an elevated protein phase forms in the inlet zone. This protein phase is a high-viscosity biphasic matrix, which is periodically entrained into the contact forming a thick protective hydro-gel film. One of the main findings of this study is that film thickness was very sensitive to load; to a much greater extent than predicted by EHL models. Thus film formation in MoM hip joints is very susceptible to high contact pressures which might be due to implant misalignment and edge-loading.
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