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.     

Analysis of biphasic lubrication of articular cartilage loaded by cylindrical indenter

Combination of theoretical biphasic analyses and corresponding experimental measurements for articular cartilage has successfully revealed the fundamental material properties and time-depending mechanical behaviors of articular cartilage containing plenty of water. The insight of load partitioning between solid and fluid phases advanced the prediction of the frictional behavior of articular cartilage. One of the recent concerns about biphasic finite element (FE) analysis seems to be a dynamic and physiological condition in terms of mechanical functionality as a load-bearing for articular joint system beyond material testing, which has mainly focused on time-dependent reaction force and deformation in relatively small and low speed compression. Recently, the biphasic FE model for reciprocating sliding motion was applied to confirm the frictional effect on the migrating contact area. The results indicated that the model of a cylindrical indenter sliding over the cartilage surface remarkably sustained the higher proportion of fluid load support than a condition without migrating contact area, but the effectiveness of constitutive material properties has not been sufficiently evaluated for sliding motion. In our present study, at the first stage, the compressive response of the articular cartilage was examined by high precision testing machine. Material properties for the biphasic FE model, which included inhomogeneous apparent Young's modulus of solid phase along depth, strain-dependent permeability and collagen reinforcement in tensile strain, were estimated in cylindrical indentation tests by the curve fitting between the experimental time-dependent behavior and FE model simulation. Then, the biphasic lubrication mechanism of the articular cartilage including migrating contact area was simulated to elucidate functionality as a load-bearing material. The results showed that the compaction effect on permeability of solid phase was functional particularly in the condition without the migrating contact area, whereas in sliding condition the compaction effect did not clearly show its role in terms of the proportion of fluid load support. The reinforcement of solid phase, which represented the collagen network in the tissue, improved the proportion of fluid load support especially in the sliding condition. Thus, a functional integration of constitutive mechanical properties as a load-bearing was evaluated by FE model simulation in this study.     

Experimental and numerical investigation of the behaviour of articular cartilage under shear loading—Interstitial fluid pressurisation and lubrication mechanisms

In this paper, the mechanical and frictional responses of articular cartilage when subjected to alternating shearing forces under a constant load were investigated. Shear testing was performed at physiological contact pressures to ascertain the influence of interstitial fluid support on the evolution of frictional forces during cyclic loading.Numerical studies were also performed using the finite element software Abaqus. The tissue was modelled as a biphasic material with strain dependent permeability. The influence of the material characteristics on the lubrication mechanisms occurring when cartilage is subject to compression and shear was studied to corroborate the experimental findings.     

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。     

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.     

仿生润滑技术研究及其应用探讨

介绍了仿生润滑技术在仿生减阻、人工关节润滑、生物医用润滑剂方面的基础研究工作与部分应用成果，讨论了仿生润滑技术未来的应用发展方向。     

Tribology of human and artificial joints.

Studies of human joint lubrication mechanisms have led to the conclusion that under normal healthy conditions they are fluid film lubricated. The main features responsible for allowing this mechanism to operate are the dynamic nature of the loading and the compliance of the bearing surfaces (articular cartilage). In contrast, artificial joints, being made of much more rigid materials, have been demonstrated to be lubricated by a mixed regime, where some load is carried by the fluid film and some by solid to solid contact. Since some surface contact takes place then wear remains a problem and friction is much higher than in human joints. The use of compliant surface bearings for artificial joints has been explored and shown to be of great advantage, reproducing the effects of natural joints. However, elastomeric materials are known to degrade in aqueous solutions so this aspect has been examined to ensure a reasonable life in the human body. Joints of the lower limb--hip, knee, and ankle--have similar load and motion patterns and behave in a similar way in terms of lubrication. Joints of the hand are not in any way similar in their behaviour and so a typical upper limb joint, the finger, has been studied to see if improvements can be made to the design of replacement artificial joints. Novel suggestions like plastic on plastic joints have been shown to be an alternative which is worthy of further consideration.     

Directed Ion Transport as Virtual Cause of Some Facilitated Friction–Lubrication Mechanism Prevailing in Articular Cartilage: A Hypothesis

Based on certain characteristics of the acid–base quasi-equilibria and on structural properties of the synovial inhomogeneous fluid in a model articular cartilage (mAC), we try to hypothesize on its facilitated friction–lubrication mechanism. We opt for a scenario that under departure from the acid–base, pH-dependent equilibrium, directed transport of protons (H⁺) is plausible, leading to a certain synergistic kinetic–thermodynamic pathway of the system as a whole. It can be viewed in such a way that protons, and virtually, other ions such as OH⁻; Ca²⁺, may pass through the (intra)micellar, possibly elongated spaces, playing their roles as if they were transported along temporarily formed ion (mainly, H⁺) transmembrane channels. Such a hypothetical scenario would thoroughly contribute to some electrostatics-aided, interstitial (synovial) biofluid pressurization, often reported by experimentalists as the appropriate mechanism of facilitating the lubrication in a real articular cartilage (rAC) in microrheological conditions, encountered in articulating joints of mammals.     

Boundary Lubrication in Natural Articular Joints

This review concentrates on studies into the behaviour of natural articular cartilage under boundary lubrication. This includes investigations into the chemical composition at the surface of cartilage, carried out as a means of identifying the boundary lubricant. Studies on the friction of cartilage sliding against cartilage and cartilage sliding steel or glass under conditions expected to be in the boundary regime are described. Additionally, model studies on the possible mechanisms of boundary lubrication using well-defined artificial surfaces are also discussed. Although there appears to be some contradiction between the results of friction measurements, an explanation can, at least in part, be given in terms of the layer of cartilage that is being measured. The different chemical nature and lubricating behaviour of the layers found at or near the surface are discussed in relation to the various results given in the literature.     

Development of artificial articular cartilage

Attempts have been made to develop an artificial articular cartilage on the basis of a new viewpoint of joint biomechanics in which the lubrication and load-bearing mechanisms of natural and artificial joints are compared. Polyvinyl alcohol hydrogel (PVA-H), 'a rubber-like gel', was investigated as an artificial articular cartilage and the mechanical properties of this gel were improved through a new synthetic process. In this article the biocompatibility and various mechanical properties of the new improved PVA-H is reported from the perspective of its usefulness as an artificial articular cartilage. As regards lubrication, the changes in thickness and fluid pressure of the gap formed between a glass plate and the specimen under loading were measured and it was found that PVA-H had a thicker fluid film under higher pressures than polyethylene (PE) did. The momentary stress transmitted through the specimen revealed that PVA-H had a lower peak stress and a longer duration of sustained stress than PE, suggesting a better damping effect. The wear factor of PVA-H was approximately five times that of PE. Histological studies of the articular cartilage and synovial membranes around PVA-H implanted for 8-52 weeks showed neither inflammation nor degenerative changes. The artificial articular cartilage made from PVA-H could be attached to the underlying bone using a composite osteochondral device made from titanium fibre mesh. In the second phase of this work, the damage to the tibial articular surface after replacement of the femoral surface in dogs was studied. Pairs of implants made of alumina, titanium or PVA-H on titanium fibre mesh were inserted into the femoral condyles. The two hard materials caused marked pathological changes in the articular cartilage and menisci, but the hydrogel composite replacement caused minimal damage. The composite osteochondral device became rapidly attached to host bone by ingrowth into the supporting mesh. The clinical implications of the possible use of this material in articular resurfacing and joint replacement are discussed.     

A rational human joint friction test using a human cartilage-on-cartilage arrangement

Efficient lubrication is essential for synovial joint mobility in both health and disease. It is well known that extremely low friction is required for proper functioning of synovial joints. In several medical treatments, bio-lubricants are injected into human joints to maintain their proper functioning. In the course of developing and screening such bio-lubricants, it is important to measure their effect under conditions similar to the ones in vivo. To this end, a first attempt was made to test the friction of two slices of human articular cartilage sliding over each other under various working conditions in the presence of different lubricating fluids. The results can be used for future research in the field of joint lubrication.     

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.     

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.     

Research on swing friction lubrication mechanisms and the fluid load support characteristics of PVA–HA composite hydrogel

Hydrogel has been extensively studied for use as articular cartilage. This study aims to investigate fluid load support mechanism of polyvinyl alcohol–hydroxyapatite composite hydrogel. Finite element method is used to study swing friction lubrication mechanism and fluid load support. The friction coefficient increases with contact load and swing angle. The fluid flow has an important effect on the fluid load support, which decreases with an increase in contact load and swing angle. The fluid load support is very high (85%), and the hydrogel has low friction coefficient. It exhibits biphasic and self-generating lubrication mechanism.     

Elastohydrodynamic and micro-elastohydrodynamic lubrication

Elastohydrodynamic lubrication is the dominant mode of lubrication in many critical, highly stressed machine elements such as gears, rolling bearings, cams and followers. It also governs the effective operation of many highly deformable or ‘soft’ bearing systems such as elastomeric bearings, seals and synovial joints. The major developments in the understanding of this exciting mode of fluid film lubrication during the latter half of the twentieth century is reviewed and attention is drawn to continuing topics of investigation.     

The effect of glycosaminoglycan depletion on the friction and deformation of articular cartilage

Glycosaminoglycans (GAGs) have been shown to be responsible for the interstitial fluid pressurization of articular cartilage and hence its compressive stiffness and load-bearing properties. Contradictory evidence has been presented in the literature on the effect of depleting GAGs on the friction properties of articular cartilage. The aim of this study was to investigate the effect of depleting GAGs on the friction and deformation characteristics of articular cartilage under different tribological conditions. A pin-on-plate machine was utilized to measure the coefficient of friction of native and chondroitinase ABC (CaseABC)-treated articular cartilage under two different models: static (4 mm/s start-up velocity) and dynamic (4 mm/s sliding velocity; 4 mm stroke length) under a load of 25 N (0.4 MPa contact stress) and with phosphate-buffered saline as the lubricant. Indentation tests were carried out at 1 N and 2 N loads (0.14 MPa and 0.28 MPa contact stress levels) to study the deformation characteristics of both native and GAG-depleted cartilage samples. CaseABC treatment rendered the cartilage tissue soft owing to the loss of compressive stiffness and a sulphated-sugar assay confirmed the loss of GAGs from the cartilage samples. CaseABC treatment significantly increased (by more than 50 per cent) the friction levels in the dynamic model (p < 0.05) at higher loading times owing to the loss of biphasic lubrication. CaseABC treatment had no effect on friction in the static model in which the cartilage surfaces did not have an opportunity to recover fluid because of static loading unlike the cartilage tissue in the dynamic model, in which translation of the cartilage surfaces was involved, ensuring effective biphasic lubrication. Therefore the depletion of GAGs had a smaller effect on the coefficient of friction for the static model. Indentation tests showed that GAG-depleted cartilage samples had a lower elastic modulus and higher permeability than native tissue. These results corroborate the role of GAGs in the compressive and friction properties of articular cartilage and emphasize the need for developing strategies to control GAG loss from diseased articular cartilage tissue.     

Compliant-poroelastic lubrication in cartilage-on-cartilage line contacts

ABSTRACT

The mechanisms of friction in natural joints are still relatively unknown and attempts at modelling cartilage-cartilage interfaces are rare despite the huge promise they offer in understanding bio-friction. This article derives a model combining finite strain, porous and thin-film flow theories to describe the lubrication of cartilage-on-cartilage line contacts. The material is modelled as compliant and poroelastic in which the micro-scale fibrous structure is interstitially filled with synovial fluid. This fluid flows over the interface between the bodies and is coupled to pressure generated by relative motion in the thin-film region formed under load. A Stribeck analysis demonstrated that this type of contact is determinable to conventional elastic lubrication but that the friction performance is improved by this interfacial flow. Moreover, the inclusion of periodic flow conditions when contact is onset is a specific novelty which elucidates new observations in the lubrication mechanisms pertaining to natural joints.     

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.     

内燃机流体润滑技术的发展及其在我国的应用

讨论了内燃机摩擦学研究中流体润滑技术在润滑模型、计算方法、润滑剂、润滑表面和润滑工况的监测等方面近年来取得的成果和研究进展；通过与主要发达国家内燃机流体润滑技术研究和应用的对比分析，阐述了目前国内内燃机流体润滑研究、应用和人才培养中存在的差距和遇到的问题。指出先进润滑系统模型及求解方法、性能良好的润滑剂、润滑表面和润滑工况的监测的研究仍是今后内燃机润滑技术研究的重点。在此基础上，结合世界相关内燃机工业的发展趋势，预测了新世纪内燃机流体润滑技术的两个新的发展方向。