The effects of proteins on the friction and lubrication of artificial joints

The tribological testing of artificial hip and knee joints in the laboratory has been ongoing for several decades. This work has been carried out in an attempt to simulate the loading and motion conditions applied in vivo and, therefore, the potential for the success of the joint. However, several different lubricants have been used in these tests. The work documented in this paper compares results obtained using different lubricants and makes suggestions for future work. Hip joints and knee joints of different material combinations were tested in a friction simulator to determine their friction and lubrication properties. Both carboxymethyl cellulose (CMC) fluids and bovine serum (with CMC fluids added) were used as the lubricants. These were prepared to various viscosities to produce the Stribeck plots. Human synovial fluid, of just one viscosity, was used as the lubricant with some of the joints to give a true comparison with physiological lubricants. The results showed that, in most cases, the lubricant had a significant effect on the friction developed between the joint surfaces. This is thought to be due to the proteins that are present within the bovine serum adsorbing to the bearing surfaces, creating 'solid-like' films which rub together, protecting the surfaces from solid-to-solid contact. This would be beneficial in terms of wear but can either increase or decrease the friction between the contacting surfaces. It is important to simulate the conditions in vivo as closely as possible when testing these joints to try to obtain a better comparison between the joints and to simulate more accurately the way that these joints will operate in the body. In an attempt to simulate synovial fluid, bovine serum seems to be the most popular lubricant used at present. It would be beneficial, however, to develop a new synthetic lubricant that more closely matches synovial fluid. This would allow us to predict more accurately how these joints would operate long-term in vivo.     

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.     

Biological reactions to wear debris in total joint replacement

The vast majority of total hip prostheses currently implanted consist of a hard metal or ceramic femoral head articulating against an ultra-high molecular weight polyethylene (UHMWPE) acetabular cup. Over the last 10 years, evidence has accumulated to show that these prostheses are prone to failure due to late aseptic loosening and few survive beyond 25 years. With an increasing need to implant hip prostheses in the younger, more active patient the need to understand the mechanisms of failure and to develop artificial hip joints using alternative materials have become major issues in the orthopaedic community. This review focuses initially on our current understanding of the biological reactions to UHMWPE prosthetic wear debris in vivo and in vitro since this is believed to be the main cause of late aseptic loosening. While the precise mechanisms of osteolysis induced by UHMWPE wear debris have not been elucidated, the major message to emerge is that it is not the wear volume that determines the biological response to the debris, but the concentration of the wear volume that is within the critical size range (0.2-0.8 micron) for macrophage activation. The review then considers whether the problem of wear-debris-induced osteolysis may be overcome with the use of new generation metal-on-metal or ceramic-on-ceramic prostheses. For metal-on-metal prostheses, the prospects for increasing the osteolysis free life of the implant are good but additional biological problems associated with the nanometre size and reactivity of the wear particles in vivo may emerge. For the ceramic-on-ceramic prostheses, although initial prospects are encouraging, more data are needed on the characteristics of the wear particles generated in vivo before predictions can be made. It is concluded that the pre-clinical testing of any new materials for joint replacement must include an analysis of the wear particle characteristics and their biological reactivity in addition to the usual assessment of wear.     

A comparative joint simulator study of the wear of metal-on-metal and alternative material combinations in hip replacements

While total hip replacement represents the major success story in orthopaedic surgery in the twentieth century, there is much interest in extending even further, early in the twenty first century, the life of implants. Osteolysis has been identified as a major factor limiting the life of prostheses, with indications that fine polyethylene wear debris, generated primarily at the interface between the femoral head and the acetabular cup, promotes the process. There is therefore considerable interest in the introduction of alternative wear resistant systems to limit the deleterious effects of wear. These alternatives include ceramic-on-ceramic and metal-on-metal configurations and the present paper is primarily concerned with the latter. Some six pairs of new metal-on-metal implants of 36 mm diameter and four pairs of existing metal-on-metal implants of 28 mm diameter were tested in a ten-station hip joint simulator in the presence of a 25 per cent bovine serum solution. The implants were tested in the anatomical position to 5 x 10(6) cycles. The new heads and cups were manufactured from CoCrMo alloy with careful attention being paid to sphericity and surface finish of both components. The wear performance of the new and existing metal-on-metal total hip replacements have been evaluated and compared. The overall wear rates have then been compared with previously reported wear rates for a zirconia-on-polyethylene prosthesis of 22 mm diameter tested on the same simulator. The comparison is taken further by recalling published penetration data for metal-on-polyethylene implants of 22 and 28 mm diameter and converting these to volumetric wear rates. It was found that the heads and cups in metal-on-metal joints wore by almost equal amounts and that the opposing surfaces converged to similar surface roughness as the testing time increased. Steady state wear rates were generally achieved after 1-2 x 10(6) cycles. The mean long-term wear rates for the metal-on-metal prostheses were very low, being 0.36 mm3/10(6) cycles and 0.45 mm3/10(6) cycles for the new implants of 36 mm diameter and established implants of 28 mm diameter respectively. These wear rates compare with 6.3 mm3/10(6) cycles for zirconia-on-ultra-high molecular weight polyethylene tested on the same simulator and representative clinical values for metal-on-polyethylene of 36 mm3/year for heads of 22 mm diameter and a reported range of 60-180 mm3/year for 28 mm heads. These values do not translate directly into numbers of particles, since the metallic debris from metal-on-metal joints is very fine. The number of metallic particles may exceed the number of polyethylene wear particles from an otherwise similar metal-on-polyethylene joint by a factor of 10(3). A detailed discussion of the size and morphology of wear debris and tissue reaction to various forms of debris is beyond the scope of this paper, but the biological response to polymeric, metallic and ceramic wear debris forms a major subject for further study. The present investigation nevertheless confirms the potential of carefully designed and manufactured metal-on-metal total replacement joints for the treatment of diseased and damaged hips.     

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.     

Start up and steady state friction of alumina against alumina

Using a pin-on-disc testing machine, the authors investigated the start up and steady state friction of highly polished alumina against itself and studied the factors affecting the friction. The lubricants used in this paper were 1 wt.% water solution of carboxymethyl cellulose sodium salt (CMC-Na 1 wt.% water solution) and distilled water. It is observed that many factors, such as lubricants, load, sliding velocity, resting time and start up acceleration etc., have an influence on the friction of alumina.against alumina. The experimental results indicate that the coefficients of the start up and steady state friction when CMC-Na 1 wt.% water solution is the lubricant are much smaller than when distilled water us used, and it is possible that alumina-on-alumina hip joints may enjoy full fluid film lubrication with good machining, good fit and a proper lubricant.     

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.     

Influence of protein and lipid concentration of the test lubricant on the wear of ultra high molecular weight polyethylene

To gain a better understanding of the ultra-high molecular weight polyethylene (UHMWPE) wear mechanism in the physiological environment, the effects of protein and lipid constituents of synovial fluid on the specific wear rate of UHMWPE were examined experimentally. The multidirectional sliding pin-on-plate wear tester was employed to simulate the simplified sliding condition of hip joint prostheses. Bovine serum γ-globulin and synthetic l--DPPC were used as model protein and lipid constituents of synovia, respectively. Results of the wear test indicated that the UHMWPE wear rate primarily depended on the protein concentration of the test lubricant. Lipids acted as a boundary lubricant and reduced polyethylene wear in the low protein lubricants. However, the polyethylene wear rate increased with increasing lipid concentrations if the protein concentration was within the physiological level. Increased interactions between protein and lipid molecules and lipid diffusion to polyethylene surface might be responsible for the increased wear.     

自乳化酯在全合成切削液中的性能研究

为探讨自乳化酯作为全合成切削液主润滑剂的可能性,以聚醚、四聚蓖麻油酸酯、自乳化酯为润滑剂调配了不同的合成切削液;采用四球抗磨试验机以及攻丝扭矩试验机、液体循环泡沫试验机,考察3种润滑剂在全合成切削液中的润滑性能、抗磨性能及抗泡性能。结果表明:自乳化酯能有效提高全合成切削液的润滑性能和抗磨性能,其润滑性能及抗磨性能与四聚蓖麻油酸酯相当;自乳化酯的抗泡性能显著优于四聚蓖麻油酸酯,与聚醚类润滑剂抗泡性能相当。自乳化酯具有良好的润滑及抗磨作用,且泡沫相对较低,可以与其他类型添加剂复配,用于全合成切削液中。     

人工关节材料的表面润滑设计与应用

人工关节的摩擦磨损问题仍然是基础研究中最重要的问题,借助表面改性技术改善假体的摩擦学性能是人工关节未来发展的必经之路。从润滑角度考虑,对假体关节材料摩擦性能的研究主要集中在表面功能化润滑结构设计以及新型仿生润滑剂研究两方面。针对功能化润滑结构,介绍表面织构设计以及聚合物刷的应用,分析表面织构参数对不同运动工况下摩擦副摩擦性能的影响,阐述表面织构的润滑机制;总结不同种类的聚合物刷结构对摩擦体系耐磨性能的调控,阐明"刷型"结构在摩擦界面的水润滑特点,提出环境介质对聚合物刷结构及性质的影响作用。针对关节润滑剂,介绍传统的关节滑液组分向聚合物仿生润滑剂的拓展。指出微/纳结构的嵌套设计与协同润滑以及润滑剂结构仿生与功能仿生的结合,将是未来的重要发展方向。     

人工关节材料生物摩擦声学测试研究

硬相对硬相人工关节因其低磨损率的特点,更为适用于日常活动量较大的年轻患者群体。而在该类关节的术后随访中发现,伴随人体的运动,一部分人工关节会由于摩擦而产生异样噪声,临床发生率为0.7%～20.9%。该现象成因复杂,涉及材料摩擦磨损、不良润滑、人工关节失配以及边缘载荷等因素。针对硬相人工关节在服役中的异响问题,提出生物摩擦学与声学交叉研究方法,建立了人工关节材料的生物摩擦声学测试系统,主要包括人工关节材料摩擦学测试装置、类消声室、以及声压信号测试采集系统,能够实现人工关节材料摩擦学与声学参数的在线测量。基于该系统试验研究ZrO2陶瓷关节材料在去离子水与仿生关节滑液润滑状态下的摩擦声学表现,发现在仿生关节液下得到较好的润滑,存在薄膜润滑情况,摩擦表面呈抛光迹象,声压及声功率值均小于去离子水润滑条件下,且与摩擦因数有较好吻合,异响频率与临床发现更为接近。通过该系统的建立与试验验证,初步探索了人工关节材料的生物摩擦声学的测试方法。     

A New Tribological Approach for Lubricated Sliding Contact of Pitted Metallic Curvature Cup

Interest in the development and application of plant-based lubricants for medical use is increasing. This study investigates palm oil lubricants as environmentally friendly and renewable resources to optimize the motion in an ergonomic simulated metal hip prosthesis with modification to the acetabular cup surface. Although metal hip replacements are extensively used, minimizing metal-on-metal friction and wear using safe lubricants requires further investigation. The main physical properties of palm kernel oil and palm fatty acid distillate are considered. The viscosity, wear scar, and coefficient of friction are compared to hyaluronic acid. A modified pin-on-disc tribometer simulates friction and wear on a 28-mm-diameter acetabular cup and microscopy image analysis is used to examine the wear scar. The physical properties of palm oil derivatives reduce friction and wear. In brief, the most significant results of this study include the effect of lubricant and number of pits on wear and friction coefficient. The contribution of this research work is to maintain stability and increase the lifetime of ergonomic metal hip implants.     

Tribological Testing of Hemispherical Titanium Pin Lubricated by Novel Palm Oil: Evaluating Anti-Wear and Anti-Friction Properties

In this study, the properties of hip implant material and lubricants were examined using a pin on disc apparatus, to compare the effect of metal-on-metal(Mo M)contact with a bio-lubricant derived from palm oil. The behaviour of the lubricants was observed during the experiments, in which a hemispherical pin was loaded against a rotating disc with a groove. A titanium alloy was used to modify the hemispherical pin and disc. Before and after the experiments, the weight and surface roughness were analysed, to detect any degradation. The results were compared according to the different kinematic viscosities.The wear rates and level of friction with each lubricant were also examined. The lubricant with the highest viscosity had the lowest frictional value. Therefore, developing suitable lubricants has the potential to prolong the lifespan of prostheses or implants used in biomedical applications. The experiments collectively show that lubricants derived from palm oil could be used as efficient bio-lubricants in the future.     

Challenges associated with using bovine serum in wear testing orthopaedic biopolymers

For appropriate in vitro wear testing of prostheses and their biomaterials, the choice of lubricant is critical. Bovine serum is the lubricant recommended by several international standards for wear testing artificial joints and their biomaterials because the wear rate and wear mechanisms closely match clinical results of polyethylene bearings. The main problem with the use of bovine serum as a lubricant is protein degradation and precipitation formation, effects that are recognized as having a direct impact on wear processes. Hence, some researchers have questioned the validity of using bovine serum in simulator testing. This paper reviews the various lubricants used in laboratory wear studies and also the properties of the synovial fluid that the lubricant is trying to replicate. It is clear from the literature survey that the composition of bovine-serum-based lubricants does not match that of synovial fluid. In view of this conclusion, it is suggested that there is a need to develop an alternative lubricant that can replace bovine serum.     

Friction and wear mechanisms in hip prosthesis: Comparison of joint materials behaviour in several lubricants

In this work, the most commonly used joint materials for substitution of hip joints, ultra high molecular weight polyethylene (UHMWPE) for the cup, and alumina, stainless steel or CoCrMo alloy for the head, were submitted to pin-on-disk tribological tests. The interfacial behaviour prosthetic material/lubricant was investigated through wettability measurements. Four lubricants were used: Hanks’ balanced salt solution (HBSS) and solutions of bovine serum albumin (BSA), of hyaluronic acid (HA) and of both components (BSA + HA) in HBSS. It was observed that the friction coefficient increased significantly with time, in a random way, when the lubricant did not contain albumin. The addition of HA induced a slight reduction in the initial value of the friction coefficient but did not affect qualitatively its behaviour for longer times. Observation of the worn metallic surfaces revealed parallel grooves typical of abrasive wear and also lumpy transfer film of UHMWPE, while the transfer of polymer to alumina was incipient. Wear was particularly intense on the steel surface where delamination and fatigue wear mechanisms were also found. In the presence of albumin, the friction coefficient remained constant and small for the metallic surfaces and increased in the case of alumina. The observation of the worn surfaces showed that the presence of albumin in the lubricant avoided the transfer of polymer for the metallic surfaces but not for the alumina surfaces. These results, together with the wettability measurements, strongly indicate that the presence of albumin in the lubricant avoids the adhesion and transfer of UHMWPE only for the least hydrophilic surfaces, which are the metallic ones.     

石墨干膜润滑剂在碳钢表面摩擦磨损性能试验研究*

传统油或脂润滑剂在极端工况环境下无法满足碳钢类零件的减摩要求,采用干膜润滑剂是提高极端工况环境下碳钢表面摩擦磨损性能的可行性方法。采用超声波分散方法制备以石墨粉末为基体的干膜润滑剂,使用压力喷涂技术使其沉积在碳钢试件表面,在端面摩擦试验仪中开展干摩擦和石墨干膜润滑剂润滑下摩擦磨损性能对比性试验研究。试验结果表明：石墨干膜润滑剂在碳钢表面的沉积效果较好,沉积的石墨干膜润滑剂具有较好的润滑性能,可以有效地保护碳钢表面不被过度磨损;喷涂石墨干膜润滑剂的碳钢试件的工作寿命随着压力载荷和主轴转速的增大而缩短,负载和滑动速度的联合作用会加速涂层向稳定方向的过渡;磨损过程中形成的微观润滑剂颗粒会形成颗粒流润滑,适当添加石墨颗粒粉末可能会延长润滑剂正常发挥减摩作用的时间。制备的石墨干膜润滑剂为碳钢在极端工况环境下的减摩提供了支持。     

Biotribocorrosion of CoCrMo orthopaedic implant materials—Assessing the formation and effect of the biofilm

Due to the renewed interest in hard-on-hard hip replacement, especially metal-on-metal (MoM) or metal-on-ceramic (MoC) joints, issues relating to their long-term durability need to be addressed. Their effects on the operating environment (human body) and how the body fluid affects the implant materials are the primary concern. For widely used metallic implant materials, such as cobalt–chromium–molybdenum (CoCrMo) alloys, released ions due to electrochemical (corrosion) processes and mechanical-enhanced electrochemical (corrosion-wear/tribocorrosion) processes may cause biological reactions in the human hosts. Proteins are a primary constituent of the synovial fluid in human joints with other organic components such as hyaluronic acid and lubricin, and, although numerous tribological studies in protein-containing fluids have been conducted, there is still a need to fully understand the role of proteins and adsorbed-protein layers in wear, corrosion and tribocorrosion processes in artificial joints.In this study, bovine calf serum was used to simulate the body fluid, and a model solution of 0.36% NaCl solution was employed to isolate the influence of organic species (such as proteins, amino acids etc.). Wrought high carbon cobalt–chromium–molybdenum alloy (HC CoCrMo), Wrought low carbon cobalt–chromium–molybdenum alloy (LC CoCrMo) and stainless steel UNS S31603 (316 L) were included in the study and their corrosion, tribology and tribocorrosion behaviour were assessed by integration of gravimetric analysis and electrochemical measurements. Surface analysis (chemical and topographical) was carried out to fully understand the surface/organic species interactions.The constituents of bovine serum have been shown to have a great influence on the corrosion behaviour of all materials studied here—the mechanism of their action being to accelerate ion release and passive film breakdown in static conditions. In tribological contacts, biofilm can play a role in forming an effective lubricating film that reduces friction. For HC CoCrMo, reactions at the surface in the contact zone form a very complex nanostructured layer which comprises wear debris, biofilm and reaction products and the process also changes the nature of the passive film formation. The film reduces the material loss and hence has a protective nature. Organic species (proteins, etc.) were also shown to enhance corrosion-related damage on all materials.     

The wear of metal-on-metal total hip prostheses measured in a hip simulator

New generation metal-on-metal prostheses have been introduced to try and overcome the problem of osteolysis often attributed to the wear particles of the polyethylene component of conventional metal-on-ultra-high molecular weight polyethylene (UHMWPE) joints. The wear rates of four metal-on-metal joints (two different clearances) were assessed along with that of a conventional metal-on-UHMWPE joint. Friction measurements of the metal-on-metal joints were taken before and after the wear test and compared. Two distinct wear phases were discernible for all the metal-on-metal joints: an initial wear phase up to 0.5 x 10(6) cycles and then a lower steady state wear phase. The steady state wear rate of the 22 microm radial clearance metal-on-metal joint was lower than that for the 40 microm radial clearance joint, although this difference was not found to be significant (p > 0.15). The wear rates for all the joints tested were consistent with other simulator studies. The friction factors produced by each joint were found to decrease significantly after wear testing (p < 0.05).     

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.     

固液两相流体弹流润滑研究

应用微极流体理论,考虑流体的可压缩性,建立线接触微极流体动力润滑的基本方程,进行固液两相流体稳态流动弹流润滑数值分析,获得了润滑油膜压力、形状以及摩擦力分布,分析了微极参数对润滑性能的影响,并与不可压缩流体结果进行比较。结果表明,固液两相润滑流体较单相牛顿流体,在增加膜厚、提高承载力方面有显著的作用,而接触表面的摩擦因数有所降低,流体的可压缩性降低了油膜压力与油膜厚度。