Comparison of friction and lubrication of different hip prostheses

It is well documented that an important cause of osteolysis and subsequent loosening of replacement hip joints is polyethylene wear debris. To avoid this, interest has been renewed in metal-on-metal and ceramic-on-ceramic prostheses. Various workers have assessed the lubrication modes of different joints by measuring the friction at the bearing surfaces, using different lubricants. Measurements of friction factors of a series of hip prostheses were undertaken using carboxymethyl cellulose (CMC) fluids, silicone fluids, synovial fluid and different concentrations of bovine serum as the lubricant. The experimental results were compared with theoretical predictions of film thicknesses and lubrication modes. A strong correlation was observed between experiment and theory when employing CMC fluids or silicone fluids as the lubricant. Mixed lubrication was found to occur in the metal-on-metal (CoCrMo/CoCrMo) joints with all lubricants at a viscosity within the physiological range. This was also the case for the metal-on-plastic (CoCrMo/ultra-high molecular weight polyethylene) joints. The ceramic-on-ceramic (Al2O3/Al2O3) joints, however, exhibited full fluid film lubrication with the synthetic lubricants but mixed lubrication with the biological lubricants. Employing a biological fluid as the lubricant affected the friction to varying degrees when compared with the synthetic lubricants. In the case of the ceramic-on-ceramic joints it acted to increase the friction factor tenfold; however, for the metal-on-metal joints, biological fluids gave slightly lower friction than the synthetic lubricants did. This suggests that, when measuring friction and wear of artificial joints, a standard lubricant should be used.     

Metal-on-metal hip simulator study of increased wear particle surface area due to 'severe' patient activity

This study investigated changes in metal-on-metal (MOM) hip wear and wear particle characteristics arising from a more aggressive patient activity level compared with normal walking. The test hypothesis was that 'severe'-gait conditions will change wear, wear particle sizes, and morphology owing to a decline in joint lubrication. Four carbon MOM hip bearings 40 mm high were subjected to normal-walking and fast-jogging simulations in an orbital hip joint simulator with 25 per cent alpha-calf serum as a lubricant. Co-Cr-Mo wear particles were extracted using an enzymatic method, and prolate ellipsoid equations were used to estimate particle volume and surface area. Fast-jogging simulations generated a sevenfold increase in volumetric wear, a 33 per cent increase in mean wear particle size, and a threefold increase in the number of larger (needle) particles compared with walking. This resulted in a twentyfold increase in total wear particle surface area per 10(6) cycles compared with walking, thereby confirming our hypothesis. The clinical significance of this result suggests that highly active MOM patients may exhibit greater ion release than less active patients.     

Determination of contact area in 'cushion form' bearings for artificial hip joints

Current forms of artificial hip joints produce wear debris, which contributes to loosening of the prostheses. These 'hard' joints articulate with boundary or mixed lubrication, whereas the natural joint articulates with full fluid film lubrication. An artificial joint that articulates with full fluid film lubrication could greatly reduce wear and frictional torque and hence reduce the incidence of loosening and inflammatory tissue reaction. The use of a thin lining of low elastic modulus in the acetabular cup is one possible way of promoting full fluid film lubrication. In the design of such cushion forms of bearings, it is important to be able to predict the contact area, stress distribution and film thickness. This paper presents experimental techniques to determine the contact area in low elastic modulus layers and compares these measured areas with theoretical predictions using linear elasticity theory. At low loads experimental results and theoretical predictions were close. However, at loads above 300 N, the theory overestimated the width of the contact area by up to 8.5 per cent. This difference is mainly attributed to the non-linear behaviour of the elastomer at the higher levels of stress.     

Analysis of mixed lubrication mechanism in metal-on-metal hip joint replacements

A simple mixed lubrication model has been developed to predict the asperity contact and wear for the metal-on-metal bearing couple for total hip joint replacements. It has been shown that the femoral head radius has a large effect on the predicted asperity contact and wear depending on the lubrication regime. An increase in the femoral head radius can lead to an increase in wear under a predominantly boundary lubrication regime, but this trend can be reversed under a mixed lubrication regime towards fluid film lubrication. These observations are consistent with the recent experimental findings from hip simulator studies by Smith and co-workers.     

Lubrication and wear modelling of artificial hip joints: A review

The tribological performance of artificial hip joints is a critical issue for their success, because adverse tissue reaction to wear debris causes loosening and failure. Many studies on wear and lubrication of hip prostheses have been published in the last 10 years, mostly on experimental tests. Theoretical/numerical models have been proposed for investigating geometrical and material parameters also. This paper reviews recent literature on lubrication and wear models, stressing simplifying hypotheses, input data, methods and results. It is pointed out that actually lubrication and wear are described neglecting each other while new advanced models including both aspects could be helpful.     

螺旋槽小孔节流动静压空气轴承颗粒冲蚀研究

为研究空气轴承在运转过程中细小颗粒对壁面冲蚀情况，借助三维建模工具建立动静压空气轴承模型，从连续方程、可压缩流体润滑方程、气体状态方程出发，推导出等温条件下稳态气体润滑Reynolds方程；结合离散相模型(DPM)颗粒平衡方程，分析颗粒运动轨迹和颗粒对壁面的最大冲蚀速率。借助Fluent仿真软件分析气源压力、主轴转速、粒径参数对气体轴承壁面冲蚀的影响。结果表明：随着粒径尺寸的增加，相同运行工况下，壁面冲蚀磨损速率呈现先增加后降低趋势；随着主轴转速的增加，壁面冲蚀磨损的面积在增加，但最大壁面冲蚀磨损速率在下降，同时壁面磨损面积向主轴正向旋转的方向延伸。     

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.     

Design aspects of compliant, soft layer bearings for an experimental hip prosthesis

Currently, an artificial hip joint can be expected to last, on average, in excess of 15 years with failure due, in the majority of cases, to late aseptic loosening of the acetabular component. A realistic alternative to the problem of wear in conventional joints is the introduction of bearing surfaces that exhibit low wear and operate in the full fluid-film lubrication regime. Contact analyses and friction tests were performed on compliant layer joints (metal-on-polyurethane) and the design of a prototype ovine arthroplasty model was investigated. When optimized, these components have been shown to achieve full fluid-film lubrication.     

Effect of wear of bearing surfaces on elastohydrodynamic lubrication of metal-on-metal hip implants

The effect of geometry change of the bearing surfaces owing to wear on the elastohydrodynamic lubrication (EHL) of metal-on-metal (MOM) hip bearings has been investigated theoretically in the present study. A particular MOM Metasul bearing (Zimmer GmbH) was considered, and was tested in a hip simulator using diluted bovine serum. The geometry of the worn bearing surface was measured using a coordinate measuring machine (CMM) and was modelled theoretically on the assumption of spherical geometries determined from the maximum linear wear depth and the angle of the worn region. Both the CMM measurement and the theoretical calculation were directly incorporated into the elastohydrodynamic lubrication analysis. It was found that the geometry of the original machined bearing surfaces, particularly of the femoral head with its out-of-roundness, could lead to a large reduction in the predicted lubricant film thickness and an increase in pressure. However, these non-spherical deviations can be expected to be smoothed out quickly during the initial running-in period. For a given worn bearing surface, the predicted lubricant film thickness and pressure distribution, based on CMM measurement, were found to be in good overall agreement with those obtained with the theoretical model based on the maximum linear wear depth and the angle of the worn region. The gradual increase in linear wear during the running-in period resulted in an improvement in the conformity and consequently an increase in the predicted lubricant film thickness and a decrease in the pressure. For the Metasul bearing tested in an AMTI hip simulator, a maximum total linear wear depth of approximately 13 microm was measured after 1 million cycles and remained unchanged up to 5 million cycles. This resulted in a threefold increase in the predicted average lubricant film thickness. Consequently, it was possible for the Metasul bearing to achieve a fluid film lubrication regime during this period, and this was consistent with the minimal wear observed between 1 and 5 million cycles. However, under adverse in vivo conditions associated with start-up and stopping and depleted lubrication, wear of the bearing surfaces can still occur. An increase in the wear depth beyond a certain limit was shown to lead to the constriction of the lubricant film around the edge of the contact conjunction and consequently to a decrease in the lubricant film thickness. Continuous cycles of a running-in wear period followed by a steady state wear period may be inevitable in MOM hip implants. This highlights the importance of minimizing the wear in these devices during the initial running-in period, particularly from design and manufacturing points of view.     

Recent developments in the tribology of artificial joints

A history of the tribological development of artificial joints compares how these are lubricated with the mechanisms involved in human joints. It is concluded that while healthy human joints are lubricated by fluid film action, all current artificial joints at best are lubricated by mixed lubrication and hence wear is taking place throughout the life of the prosthesis. A new concept in artificial joints is described. Soft elastic layers simulate articular cartilage and if selected carefully can develop full fluid film lubrication with consequential low friction and minimal wear.     

An in vitro wear study of alumina-alumina total hip prostheses.

Four 28 mm diameter alumina-alumina hip prostheses were tested in the Mkll Durham hip simulator for 5 x 10(6) cycles using 25 per cent bovine serum as lubricant. Wear of the heads and cups was measured gravimetrically. The mean and standard deviation of the wear rate for the alumina cups was 0.097 +/- 0.039 mm3/10(6) cycles. The femoral heads produced such low wear that it could not be measured by weighing but could be detected byincreased surface roughness measurements. Such low wear rates represent about one-five-hundredthof the wear of ultra-high molecular weight polyethylene (UHMWPE) against ceramic in a similar test and supports work which indicates that fluid film lubrication exists in these joints.     

Contact mechanics and lubrication analyses of ceramic-on-metal total hip replacements

Ceramic-on-metal (CoM) total hip replacements have shown reduced wear and friction. Lubrication and contact mechanics analyses play an important role in providing an overall understanding for the tribological performance of CoM bearings. In the present study, the steady-state contact mechanics and elastohydrodynamic lubrication (EHL) and transient EHL of CoM bearings were analyzed. The dry and lubricated contact pressures of CoM bearings showed typical characteristics of hard-on-hard hip bearings. The effects of head radius and radial clearance on the lubrication performance were predicted. CoC and CoM bearings are more likely to benefit full fluid film lubrication than MoM bearings.     

Tribological role of synovial fluid compositions on artificial joints — a systematic review of the last 10 years

Biological components of synovial fluid and their concentration play a crucial role in the lubrication mechanism of artificial joints, particularly boundary lubrication. The purpose of this review was to summarise and critically analyse the lubrication mechanism and their tribological outcomes to artificial joints. Thirteen papers published between 01/01/2003 and 28/02/2013 met the inclusion criteria for the review. Four major biological components of synovial fluid (albumin, globulin, hyaluronic acid and lubricin) were found to have an influence on film thickness, friction coefficient and wear rate. The role of these components was reported to be varied, depending on not only their composition and concentration but also surface material properties, wettability, temperature and pressure. The findings suggest that an appropriate synovial fluid composition should be represented in a simulated body fluid in order to evaluate an implant material and subsequently to conduct biotribology tests. Copyright © 2014 John Wiley & Sons, Ltd.     

Lubrication regimes in lumbar total disc arthroplasty

A number of total disc arthroplasty devices have been developed. Some concern has been expressed that wear may be a potential failure mode for these devices, as has been seen with hip arthroplasty. The aim of this paper was to investigate the lubrication regimes that occur in lumbar total disc arthroplasty devices. The disc arthroplasty was modelled as a ball-and-socket joint. Elastohydrodynamic lubrication theory was used to calculate the minimum film thickness of the fluid between the bearing surfaces. The lubrication regime was then determined for different material combinations, size of implant, and trunk velocity. Disc arthroplasties with a metal-polymer or metal-metal material combination operate with a boundary lubrication regime. A ceramic-ceramic material combination has the potential to operate with fluid-film lubrication. Disc arthroplasties with a metal-polymer or metal-metal material combination are likely to generate wear debris. In future, it is worth considering a ceramic-ceramic material combination as this is likely to reduce wear.     

Influence of polymer surface chemistry on frictional properties under protein-lubrication conditions: implications for hip-implant design

Wear processes in hip joints are believed to occur chiefly under boundary lubrication conditions. We have shown that the efficiency of boundary lubrication of the ultrahigh-molecular-weight polyethylene (UHWMPE)–alumina tribopair in protein-containing solutions can be improved by modifying the surface hydrophilicity of the UHMWPE. Our experiments show that an oxygen-plasma treatment of polyethylene, producing significantly greater hydrophilicity due to modified surface chemistry, leads to faster and modified protein adsorption. A denser boundary layer of human serum albumin (HSA) proteins on the PE surface appears to enhance boundary lubrication, which leads to a 50% reduction of dynamic friction, as well as to a reduction of stiction, which is believed to be a key factor in wear mechanisms occurring in artificial hip joints. Following tribological testing in pure water, we observed the presence of a polyethylene transfer film on the alumina disc. This film was not formed after tribotesting either in protein or in Ringer's solution.     

在刀-屑界面持续润滑刀具切削45钢的性能及润滑机理

为提高刀具润滑性能,尽量减少切削液的使用,制备出在刀屑界面持续润滑的新型刀具,能够将切削液通过微通道直接输送到刀屑接触界面内部。采用该新型刀具与普通刀具在干切削和浇注切削液条件下分别进行切削45钢试验,测量了切削三向力,对刀具前刀面磨损面进行SEM微观形貌分析及元素检测,分析了刀具的摩擦磨损特性及润滑机理。试验结果表明,与普通刀具在干切削和浇注切削液条件下相比,刀屑界面持续润滑刀具能够有效减少切削过程中的摩擦磨损,而切削液用量只有传统浇注式切削的1/120。分析前刀面的元素可知,切削液能够更加深入到离主切削刃更近的区域,并能持续供给,这是该刀具具有更好的减摩抗磨效果的主要原因。尽管新型刀具的黏结情况大大缓解,但刀具的磨损机理仍然以黏结磨损和氧化磨损为主。     

交联超高分子量聚乙烯人工髋关节磨损仿真模拟

人工髋关节超高分子量聚乙烯（UHMWPE）关节面磨损仍是影响置换关节远期寿命的主要因素,其仿真建模是对关节模拟机磨损测试手段的重要补充,也是实现置换关节临床前性能评估的有效方法。由多向运动产生的交叉剪切效应是影响UHMWPE磨损的主要原因之一,也是仿真建模的关键。现有理论方法将磨损深度确定为滑动距离的函数,并将90°交叉剪切运动条件下的磨损作为度量基准计算不同角度下的交叉剪切效应,但尚未考虑接触应力变量对磨损深度的影响。针对以上问题,提出了在垂直交叉剪切运动条件下将磨损深度表示为摩擦功函数的方法。该方法利用UHMWPE摩擦因数与接触压力的定量关系计算摩擦因数并确定摩擦功,解决了UHMWPE磨损交叉剪切效应中滑动距离与接触应力的耦合问题。基于磨损仿真新模型研究了36 mm直径的交联UHMWPE髋关节,并与已有ProSim模拟机试验结果进行了验证。结果显示该仿真模型可准确计算体积磨损和线性磨损等磨损量以及髋关节载荷方向改变对磨损的影响。磨损新模型为进一步仿真模拟奠定了有效基础。     

Soft layer lubrication of artificial hip joints

An experimental study of the lubrication mechanisms existing in prototype artificial hip joints which have compliant surface linings, has shown that for a lining with a hardness of about 4N/mm², fluid film lubrication can be achieved even with the low viscosity synovial fluid present in diseased joints after surgery. In tests, a synthetic lubricant was used in place of the synovial fluid. The best results seem to be produced with a 2 mm thick layer.     

Influence of gelatin and bovine serum lubricants on ultra-high molecular weight polyethylene wear debris generated in in vitro simulations

Ultra-high molecular weight polyethylene (UHMWPE) wear debris induced osteolysis has a major role in the late aseptic loosening and ultimate failure of total hip replacements (THR). Clinically relevant in vitro simulations of wear are essential to predict the osteolytic potential of bearing surfaces in artificial hip joints. Newborn calf or bovine serum has been accepted as a boundary lubricant for such in vitro tests, but its biological stability has been questioned. This study compared the wear factors, number of wear particles and levels of microbial contamination produced in bovine serum and a gelatin-based lubricant. The wear factors produced by the two lubricants were not significantly different, however the wear debris morphology produced was substantially different. The bovine serum became contaminated with micro-organisms within 28 h, whereas the protein-based lubricant remained uncontaminated. The results showed that bovine serum was not a stable boundary lubricant. They also showed that although the wear factors for the two solutions were not significantly different, the protein-based lubricant was not a suitable alternative to bovine serum because the wear debris produced was not clinically relevant.     

A 12-station anatomic hip joint simulator

A novel 12-station hip joint simulator with an anatomic position of the prosthesis was designed and built. The motion of the simulator consists of flexion-extension and abduction-adduction. The load is of the double-peak type. The validation test was done with three similar 28 mm CoCr-polyethylene joints in diluted calf serum lubricant for 3.3 x 10(6) cycles. The bearing surfaces of the polyethylene cups were burnished, the CoCr heads were undamaged, the wear particles were in the 0.1-1 microm size range, and the mean wear factor of the polyethylene cups was 5.7 x 10(-7) mm(3)/N m. These essential observations were in good agreement with clinical findings. In addition, three similar 50 mm CoCr/CoCr joints, representing the contemporary large-diameter metal-on-metal articulation were tested. The wear of the CoCr/CoCr joints was calculated from the Co and Cr concentrations of the used lubricant quantified with atomic absorption spectroscopy. The bearing surfaces of the CoCr/CoCr joints showed mild criss-cross scratching only. The average wear factor of polyethylene cups was 275 times that of the CoCr/CoCr joints. The tribological behaviour of the large-dia. CoCr/CoCr appeared to be dominated by fluid film lubrication, as indicated by very low frictional heating and wear, making it tribologically superior to the conventional CoCr/polyethylene, and therefore very interesting clinically. In conclusion, the simulator proved to be a valid, reliable, practical, economical, and easy-to-operate tool for wear studies of various hip replacement designs.