Importance of head diameter, clearance, and cup wall thickness in elastohydrodynamic lubrication analysis of metal-on-metal hip resurfacing prostheses

The main design features of metal-on-metal (MOM) hip resurfacing prostheses in promoting elastohydrodynamic lubrication were investigated in the present study, including the femoral head diameter, the clearance, and the cup wall thickness. Simplified conceptual models were developed, based on equivalent uniform wall thicknesses for both the cup and the head as well as the support materials representing bone and cement, and subsequently used for elastohydrodynamic lubrication analysis. Both typical first- and second-generation MOM hip resurfacing prostheses with different clearances and cup wall thicknesses were considered with a fixed large bearing diameter of 50 mm, as well as a 28 mm diameter MOM total hip replacement bearing for the purpose of comparison. The importance of the head diameter and the clearance in promoting elastohydrodynamic lubrication was confirmed. Furthermore, it was also predicted that a relatively thin acetabular cup in the more recently introduced second-generation MOM hip resurfacing prostheses would be capable of improving elastohydrodynamic lubrication even further.     

Tribological principles in metal-on-metal hip joint design

An analysis of some 100 published and communicated findings on running-in volumetric wear and steady state wear rates from simulator tests carried out in eight laboratories in three countries has been undertaken. Powerful indications have emerged of the dominant role of mixed lubrication in current metal-on-metal hip replacements, with elastohydrodynamic film thickness controlling wear. The background to the calculation of film thickness in the elastic-isoviscous mode of lubrication has been outlined and graphs of representative film thickness and lambda ratio have been presented. For minimum wear and wear rate the diameter of the femoral component should be as large as possible, while the clearance should be as small as is practicable. The findings are valid for both monolithic and surface replacement implants. A tentative proposal is made for the prediction of lifetime wear in metal-on-metal total hip replacements.     

Design considerations for cushion form bearings in artificial hip joints.

Lubrication mechanisms and contact mechanics have been analysed in a new generation of 'cushion form' bearings for artificial hip joints, which comprise low elastic modulus layers on the articulating surfaces. Comparisons have been made with 'hard' bearings used in existing prostheses and also with the natural hip joint. Lubricating film thicknesses are enhanced by larger contact areas and lower contact pressures. For a fixed contact area, simultaneous changes in layer thickness and radial clearance have been shown to have a small effect on elastohydrodynamic film thickness. Hard bearings designed with the same contact area as the cushion bearings produced a similar film thickness, but lubricant film thickness is not optimized in current designs. The main advantage of using a cushion bearing with low elastic modulus layers was found to be associated with microelastohydrodynamic lubrication. Careful selection of the elastic modulus is important in order to ensure that this lubrication regime was effective. Low elastic modulus layers may also produce local deformations, which enhance squeeze film action. The elastic modulus of the material should not be lower than necessary to produce effective microelastohydrodynamic lubrication, as a further reduction in modulus only increases the strain distribution in the material. A lubricant film thickness of 0.3 microns has been predicted for a cushion hip prosthesis with a femoral head diameter of 32 mm and radius of contact zone of 16 mm, using a 2 mm thick layer with an elastic modulus of 20 MPa.     

The tribological behaviour of different clearance MOM hip joints with lubricants of physiological viscosities

Clearance is one of the most influential parameters on the tribological performance of metal-on-metal (MOM) hip joints and its selection is a subject of considerable debate. The objective of this paper is to study the lubrication behaviour of different clearances for MOM hip joints within the range of human physiological and pathological fluid viscosities. The frictional torques developed by MOM hip joints with a 50 mm diameter were measured for both virgin surfaces and during a wear simulator test. Joints were manufactured with three different diametral clearances: 20, 100, and 200 microm. The fluid used for the friction measurements which contained different ratios of 25 percent newborn calf serum and carboxymethyl cellulose (CMC) with the obtained viscosities values ranging from 0.001 to 0.71 Pa s. The obtained results indicate that the frictional torque for the 20 microm clearance joint remains high over the whole range of the viscosity values. The frictional torque of the 100 microm clearance joint was low for the very low viscosity (0.001 Pa s) lubricant, but increased with increasing viscosity value. The frictional torque of the 200 microm clearance joint was high at very low viscosity levels, however, it reduced with increasing viscosity. It is concluded that a smaller clearance level can enhance the formation of an elastohydrodynamic lubrication (EHL) film, but this is at the cost of preventing fluid recovery between the bearing surfaces during the unloaded phase of walking. Larger clearance bearings allow a better recovery of lubricant during the unloaded phase, which is necessary for higher viscosity lubricants. The selection of the clearance value should therefore consider both the formation of the EHL film and the fluid recovery as a function of the physiological viscosity in order to get an optimal tribological performance for MOM hip joints. The application of either 25 per cent bovine serum or water in existing in vitro tribological study should also be revised to consider the relevance of clinic synovial fluid viscosities and to avoid possible misleading results.     

'Severe' wear challenge to 'as-cast' and 'double heat-treated' large-diameter metal-on-metal hip bearings

The wear generation of double-heat-treated and as-cast large-diameter metal-on-metal (MOM) hip bearings was investigated using standard- and 'severe'-gait simulations. The test hypothesis was that double heat treatment would change MOM hip wear compared with the as-cast condition. Two groups of high-carbon MOM bearings of 40 mm diameter were manufactured and subjected to either hot isostatic pressing (HIP) and solution annealing (SA) or no heat treatment (as cast). The results showed no statistical difference between the two groups under both running-in and steady state conditions. Even under the most 'severe'-gait simulation published to date, the mean volumetric wear rates were 2.9 and 3.9 mm3 per 10(6) cycles for the HIP-SA and as-cast bearings respectively, showing a ten-fold increase in wear compared with walking. These differences were not statistically different; therefore our hypothesis was negated. Changes in alloy microstructure do not appear to influence the wear behaviour of high-carbon cast MOM articulations with similar chemical compositions. This is in sharp contrast with the published significance of bearing diameter and radial clearance on the wear of MOM hip bearings.     

A finite element method comparison of wear in two metal-on-metal total hip prostheses

The contact mechanics of two metal-on-metal (MOM) total hip prostheses was studied by means of the finite element method (FEM). The purpose of the work was to compare two total hip replacements (Durom and Metasul) with regard to the amount of wear debris released. Wear on the bearing surfaces was evaluated following Reye hypotheses from the pressure distribution, computed by means of three-dimensional FEM models; an approximate analytical model based on Hertz contact theory has also been developed and discussed. The results show that in the dry friction condition the Durom joint releases almost twice as much wear volume as produced by the Metasul joint. Therefore, while Durom implants can improve hip stability by increasing the prosthetic impingement-free range of motion (PIF-ROM), Metasul prostheses can be a valuable solution whenever wear represents a critical choice factor.     

The effect of 'running-in' on the tribology and surface morphology of metal-on-metal Birmingham hip resurfacing device in simulator studies

It is well documented that hard bearing combinations show a running-in phenomenon in vitro and there is also some evidence of this from retrieval studies. In order to investigate this phenomenon, five Birmingham hip resurfacing devices were tested in a hip wear simulator. One of these (joint 1) was also tested in a friction simulator before, during, and after the wear test and surface analysis was conducted throughout portions of the testing. The wear showed the classical running in with the wear rate falling from 1.84 mm3 per 10(6) cycles for the first 10(6) cycles of testing to 0.24 mm3 per 10(6) cycles over the final 2 x 10(6) cycles of testing. The friction tests suggested boundary lubrication initially, but at 1 x 10(6) cycles a mixed lubrication regime was evident. By 2 x 10(6) cycles the classical Stribeck curve had formed, indicating a considerable contribution from the fluid film at higher viscosities. This continued to be evident at both 3 x 10(6) and 5 x 10(6) cycles. The surface study complements these findings.     

海水润滑赛龙陶瓷轴承的摩擦学性能研究

分析海水润滑轴承的主要磨损形式,建立海水润滑赛龙陶瓷轴承的弹流润滑模型,通过数值计算发现在赛龙陶瓷/钢摩擦副间可以形成海水弹流润滑膜,轴承间水膜厚度分布有明显颈缩现象,但压力分布图中第二压力峰不明显;随着转速的增加,海水润滑膜膜厚及最小膜厚都变薄;相同条件下,赛龙陶瓷轴承用海水润滑比用纯水和油润滑时更不容易形成弹流润滑薄膜。     

Transient elastohydrodynamic lubrication in artificial knee joint with non-Newtonian fluids

This paper presents the transient analysis of a human artificial knee joint under elastohydrodynamic lubrication (EHL) for point contact with non-Newtonian lubricants. The artificial knee joints use ultra high molecular weight polyethylene (UHMWPE) against metal with time-varying speed and load during walking. This numerical simulation employed a perturbation method, Newton Raphson method and multigrid method with full approximation technique to solve simultaneously both the time-dependent Reynolds equation, with non-Newtonian fluid based on a Carreau model, and the elasticity equation.The general numerical schemes are implemented to investigate the characteristics of elastohydrodynamic lubrication in human artificial knee joints; profiles of pressure and film thickness are determined, with varying material and lubricant properties, applied loads and speeds. The results show that the elastohydrodynamic fluid film thickness between the metallic component of the artificial knee joint and the soft polyethylene bearing becomes larger as the contact area increases and the fluid film pressure decreases. At the beginning of the first walking cycle, the film thickness is lower than in subsequent cycles because of the time required to develop the fluid film; after the first cycle, the fluid film is similar for every cycle and is dependent on transient applied load and speed during human movement.     

时变性对圆柱齿轮弹流润滑的影响

基于有限长线接触弹流理论，分析了时变性对圆柱直齿轮弹流润滑的影响，研究了动力粘度、卷吸速度与曲率半径的变化对弹性流体动力润滑油膜厚度的影响，探讨了其中的规律，得到了时变条件下齿轮弹流润滑的基本规律，提出了改进齿轮润滑效果的措施。     

An elastohydrodynamic lubrication (EHL) model of wear particle migration in an artificial hip joint

A full fluid ball-in-socket elastohydrodynamic lubrication (EHL) analysis of an artificial hip joint made of a metallic femoral head and ultra-high molecular weight polyethylene (UHMWPE) acetabular cup was considered. Since artificial hips operate in a mixed lubrication mode, wear occurs and wear particles lead to reduced hip lifetimes. This study involves simulating these particles within the lubrication regime. Hip deformation was compared to models employing finite element analysis and the spherical fast-Fourier transform technique. Particle modeling results were compared to suspension modeling experiments by other researchers. Results show a strong influence of lubricant fluid velocity on that of the wear particles.     

Steady-state elastohydrodynamic lubrication analysis of a metal-on-metal hip implant employing a metallic cup with an ultra-high molecular weight polyethylene backing

The elastohydrodynamic lubrication (EHL) analysis was carried out in this study for a 28 mm diameter metal-on-metal hip prosthesis employing a metallic cup with an ultra-high molecular weight polyethylene (UHMWPE) backing under a simple steady state rotation representing the flexion/extension during walking. Both Reynolds and elasticity equations were coupled and solved numerically by the finite difference method. The elastic deformation was determined by means of the fast Fourier transform (FFT) technique using the displacement coefficients obtained from the finite element method. Excellent agreement of the predicted elastic deformation was obtained between the FFT technique and the conventional direct summation method. The number of grid points used in the lubrication analysis was found to be important in predicting accurate film thicknesses, particularly at low viscosities representative of physiological lubricants. The effect of the clearance between the femoral head and the acetabular cup on the predicted lubricant film thickness was shown to be significant, while the effect of load was found to be negligible. Overall, the UHMWPE backing was found not only to reduce the contact pressure as identified in a previous study by the authors (Liu et al., 2003) but also significantly to increase the lubricant film thickness for the 28 mm diameter metal-on-metal hip implant, as compared with a metallic mono-block cup.     

涂层材料热特性参数对点接触弹流润滑的影响

为探究涂层材料热特性参数对点接触弹流润滑的影响,选择3种不同方法制备的类金刚石（DLC）涂层和氧化锆陶瓷涂层,构建考虑涂层热特性的点接触弹流润滑模型,分析涂层材料、涂层厚度和润滑剂的流变性对接触区润滑性能的影响。结果表明：在弹流润滑状态下具有不同热特性的4种表面涂层导致了膜厚的差异,固体表面温度及润滑区温度场会随涂层热惯性变化;热惯性最小的DLC涂层加在快速运动表面能获得更高的膜厚;随着涂层厚度的增加,会引起固体表面的温度升高,使摩擦因数降低;非牛顿流体对压力、膜厚的影响很小,但与牛顿流体相比,能获得相对较低的温度。在弹流润滑状态下,涂层覆在快表面对于减小摩擦、提高膜厚是有益的。     

Effect of wear on EHD film thickness in sliding contacts

A theoretical solution to the elastohydrodynamic (EHD) lubrication problem in sliding contacts, which takes into consideration the effect of the change in shape of the gap due to wear on the load‐carrying capacity, is presented. The model of such a contact is based on assumptions of Grubin and Ertel (von Mohrenstein). The resultant dimensionless Reynolds and film profile equations have been solved numerically for a number of cases with several values of thickness of the worn layer. Iteration of the EHD film thickness is performed by means of the secant method. Values of the calculated dimensionless film thickness are presented as a function of dimensionless wear. The conclusions concern the influence of the linear wear on the film thickness in heavily loaded sliding contacts. Copyright © 2006 John Wiley & Sons, Ltd.     

Influence of Surface Roughness on the Transfer Film Formation and Frictional Behavior of TiC/a-C Nanocomposite Coatings

Influence of surface roughness on the friction of TiC/a-C nanocomposite coatings while sliding against bearing steel balls in humid air was examined by detailed analyses of the wear surfaces and the wear scar on the ball counterparts by atomic force microscopy, optical, and confocal microscopy. It was observed that the surface roughness of the coatings essentially determines the wear behavior of the ball counterpart, which consequently influences the transfer film formation. A rough coating causes abrasive wear of the steel ball during the running-in period, which impedes the formation of a stable transfer film and leads to higher values of coefficient of friction (CoF). Moreover, the CoF does not show a decreasing trend after the running-in period, although the roughness of the coating was greatly reduced. Replacing the worn ball with a new one after the running-in period yields lower CoF values similar to that observed for a smooth coating. In both of the cases, no wear of the steel ball occurs and a stable transfer film forms and effectively covers the contact area. The influence of the wear debris on the formation of the transfer film is also discussed.     

Elastohydrodynamic lubrication analysis of metal-on-metal hip prostheses under steady state entraining motion

The elastohydrodynamic lubrication problem of metal-on-metal hip joint replacements was considered in this study. A simple ball-in-socket configuration was used to represent the hip prosthesis. The Reynolds equation in a spherical coordinate was adopted for the fluid-film lubrication analysis, to account for the ball-in-socket geometry. The corresponding elastic deformation was calculated by means of the finite element method in order to consider the complex ball-in-socket geometry as well as the backing materials underneath the acetabular cup. Both the Reynolds and the elasticity equations were solved simultaneously using the Newton-Raphson finite difference method. The general methodology developed was then applied to a recent experimental prototype metal-on-metal hip implant. It was shown that the backing materials underneath the acetabular cup had little influence on the predicted contact pressure and the elastic deformation at the bearing surfaces for this particular example. Both the film thickness and the hydrodynamic pressure distributions were obtained under various loads up to 2500 N. The predicted minimum lubricating film thickness from the present study was compared with a simple estimation using the Hamrock and Dowson formulae based upon an equivalent ball-on-plane model and excellent agreement was found. However, it was pointed out that for some forms of metal-on-metal hip prostheses with a thin acetabular cup, a polyethylene inlay underneath a metallic bearing insert or a taper connection between a bearing insert and a fixation shell, the general methodology developed in the present study should be used and this will be considered in future studies.     

Elastomer-lined bearings: An analysis of a heavily loaded cylinder sliding on a compliant layer

An analytical solution to the elastohydrodynamic lubrication of a rigid cylinder sliding on an elastic layer which is bonded to a rigid substrate is presented. The central film thickness is predicted by assuming that the lubricant film may be divided into a curved inlet region and a central tilted pad region. Expressions for pressure and deformation derived from the dry contact formulae are used to enable compatibility of pressure between the two regions to be established. The computed lubricant pressure distribution is compared with that for indentation in dry contact under the same load. The film thickness results under heavy loads indicate similar trends to those obtained by alternative solutions.     

Microprocessor Control of Running-in of Plain Bearings

Correct running-in procedures have a major influence on the subsequent life and performance of a plain bearing. With very large bearings, the process can be both time-consuming and expensive.

It is necessary to control the running-in conditions very carefully, since too thick a lubricant film, will retard the running-in, while too little lubrication will lead to overheating and possible damage or even seizure. Since the surface topography is changing continuously during running-in, the optimum lubrication condition also changes.

This paper describes a new bearing rig in which running-in parameters are monitored and fed to a microprocessor which, in turn, controls the loading and speed conditions. The objective is to obtain correct running-in, in the minimum possible time.     

Analysis of elastohydrodynamic lubrication in McKee-Farrar metal-on-metal hip joint replacement

An elastohydrodynamic lubrication (EHL) analysis was carried out in this study for a typical McKee-Farrar metal-on-metal hip prosthesis under a simple steady state rotation. The finite element method was used initially to investigate the effect of the cement and bone on the predicted contact pressure distribution between the two articulating surfaces under dry conditions, and subsequently to determine the elastic deformation of both the femoral and the acetabular components required for the lubrication analysis. Both Reynolds equation and the elasticity equation were coupled and solved numerically using the finite difference method. Important features in reducing contact stresses and promoting fluid-film lubrication associated with the McKee-Farrar metal-on-metal hip implant were identified as the large femoral head and the thin acetabular cup. For the typical McKee-Farrar metal-on-metal hip prosthesis considered under typical walking conditions, an increase in the femoral head radius from 14 to 17.4 mm (for a fixed radial clearance of 79 microm) was shown to result in a 25 per cent decrease in the maximum dry contact pressure and a 60 per cent increase in the predicted minimum film thickness. Furthermore, the predicted maximum contact pressure considering both the cement and the bone was found to be decreased by about 80 per cent, while the minimum film thickness was predicted to be increased by 50 per cent. Despite a significant increase in the predicted minimum lubricating film thickness due to the large femoral head and the thin acetabular cup, a mixed lubrication regime was predicted for the McKee-Farrar metal-on-metal hip implant under estimated in vivo steady state walking conditions, depending on the surface roughness of the bearing surfaces. This clearly demonstrated the important influences of the material, design and manufacturing parameters on the tribological performance of these hard-on-hard hip prostheses. Furthermore, in the present contact mechanics analysis, the significant increase in the elasticity due to the relatively thin acetabular cup was not found to cause equatorial contact and gripping of the ball.     

Ionic Liquid Lubrication Effects on Ceramics in a Water Environment

Ionic liquids were studied to determine their effectiveness as boundary lubricant additives for water. The chemical and tribochemical reactions that govern their behavior were probed to understand lubrication mechanisms. Under water lubricated conditions, silicon nitride ceramics are characterized by a running-in period of high friction, during which time the surface is modified causing a dramatic decrease in friction and wear. Two mechanisms have been proposed to explain the friction and wear behavior. Si₃N₄ sliding against itself may result in tribochemical reactions that form a hydrated silicon oxide layer on the surface of the sliding contact. This film has been suggested to mediate friction and wear. Others have suggested that tribo-dissolution of SiO₂ results in an ultra smooth surface and after a running-in period of high wear, the lubrication mode becomes hydrodynamic. The goal of this study was to examine the effects that ionic liquids have on the friction and wear properties of Si₃N₄, in particular their effects on the running-in period. Tribological properties were evaluated using pin-on-disk and reciprocating tribometers. The tribological conditions of the tests were selected to produce mixed/hydrodynamic lubrication. The relative lubrication mode between mixed and hydrodynamic was controlled by the initial surface roughness. Solutions containing 2 wt% ionic liquids were produced for testing purposes. Chemical analysis of the sliding surfaces was accomplished with X-ray photoelectron spectroscopy (XPS) and Fourier transform infrared spectroscopy (FTIR). The test specimens were 1 in diameter Si₃N₄ disks sliding against 1/4 in Si₃N₄ balls. The addition of ionic liquids to water resulted in dramatically reduced running-in periods for silicon nitride from thousands to the hundreds of cycles. Proposed mechanisms include the formation of BF_x and PF_x films on the surface and creation of an electric double layer of ionic liquid.