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.     

Evaluation of a superior lubrication mechanism with biphasic hydrogels for artificial cartilage

To extend the durability of artificial joints, biomimetic artificial hydrogel cartilage is proposed as a way of improving the lubrication mechanism in artificial joints. The application of hydrogels with properties similar to those of articular cartilage can be expected to duplicate the superior load-carrying capacity and lubricating ability of natural synovial joints. Frictional behaviors with three kinds of poly(vinyl alcohol) (PVA) hydrogels with high water content were examined in reciprocating tests. Interstitial fluid pressure, von Mises stress and fluid flow were compared in biphasic finite element analysis, and frictional behavior was evaluated in terms of biphasic lubrication and surface lubricity. Hybrid gel prepared by a combination of cast-drying and freeze-thawing methods showed superior low friction.     

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。     

Wear and lubrication in an artificial knee joint replacement

A theoretical study is made of the lubrication mechanism occuring in knee joint replacement under restricted motion. The idealised model has been shown to produce results, consistent with those in normal situations. Effects of increase in concentration of suspended particles in the analysis are similar to that of increase in concentration of hyaluronic acid molecules in synovial fluid. Important deductions are made for load capacity and volume wear rate and it has been shown that the slip velocity plays an important role in maintaining the self-adjusting nature of human joints.     

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.     

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.     

A Biphasic Model for Hip-Joint Replacement

The ball-and-socket geometry of the hip joint makes kinematic analysis of the joint motion relatively straightforward in comparison to other joints. The load-carrying surfaces of both ball and socket are covered with tough viscoelastic material known as cartilage. A number of lubrication theories have been proposed in the literature to account for the low coefficient of friction and low wear observed in healthy joints. The actual mechanism by which joints are capable of sustaining large repetitive loads with virtually no wear and with very little friction has not been fully understood. Therefore, analytical studies are presented for the understanding of the lubrication mechanism occurring in hip-joint replacements under restricted motion during standing or in the supporting phase during walking. The viscoelastic fluid has been considered to represent the synovial fluid in the fluid-film region. The problem described here has been analyzed in two regions (the porous matrix and the fluid-film region) separately along with suitable matching and boundary conditions at the interface. It has been concluded that the effect of the viscoelastic parameter for a particular gap is to increase the load capacity, indicating positive effects of the increase in concentration of suspended particles in the lubricant region. It has been observed that the coefficient of friction decreases with increasing values of the viscoelestic parameter. This is due to the fact that as the viscoelastic parameter increases, the concentration of hyaluronic acid molecules increases. It may also be noted from the results that the coefficient of friction decreases with increasing values of slip parameter. This shows that the slip velocity occurring at the porous boundary helps in maintaining normal functioning of human joints.     

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。     

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.     

Squeeze-film of unconformal, compliant and layered contacts

Compliant and permeable layered surfaces are used in natural synovial joints and have been proposed for the bearing surface in artificial joint replacements. The squeeze-film lubrication mechanism plays an important role in long-term performance of both natural and artificial joints. The full lubrication solution of the present problem including compliance and permeability is not available yet in the literature.The present study analyses the squeeze effect for a layered contact in two distinct cases:

• Impermeable and compliant layer for a spherical contact, based on Winkler assumption. For a spherical hip joint, yields to an optimal clearance, which gives the most persistent film.

• Porous and deformable layer with variable permeability, imbibed with lubricant, for spherical and plane contact, based on a novel mechanism, named Ex-Poro-Hydrodynamic (XPHD). The results obtained put in evidence an optimum initial compacticity for the porous matrix. The comparison with the well known model of McCutchen [Wear 1962;5:1-17] shows good agreement.

     

A model for the lubrication mechanism in knee joint replacements

Analytical studies are presented of the lubrication mechanism occurring in knee-joint replacements under restricted motion. The idealized model is shown to produce results consistent with those in normal situations. The effects of increasing the viscoelastic parameter of the lubricant are similar to those produced by an increase in the concentration of hyaluronic acid molecules in synovial fluid. Slip velocity occurring at the porous boundary aids the normal functioning of the joints. The load capacity of the replaced tibia has been shown to increase as the gap closes and/or the femoro-tibial angle decreases from π/20 to ?π/20 rad.     

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.     

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.     

Quantitative analysis of joint lubrication

A comprehensive theoretical analysis of the extent of elastohydrodynamic lubrication in human joints is presented. The analytical model is developed from existing experimental data on the geometry, loading, kinetics and elastic properties of the hip joint and the viscous properties of synovial fluid. Results of a computer-generated numerical solution of the lubrication equations are given which demonstrate that elastohydrodynamic lubrication does not persist within human joints. An alternative lubrication mechanism based on the information obtained from the analysis is discussed.     

The synovial joint as an “intelligent” friction unit

A concept is advanced and a structural diagram is shown of the synovial joint as an “intelligent” friction unit possessing the properties of a cybernetic system. The joint demonstrates a record-low friction coefficient and compensation of cartilage wear as a triboengineering material maintaining human locomotor activity for many years. It is provided with a feedback system with the sympathetic center controlling the lubricating conditions of the movable joint of the articular bone ends, the lubricating fluid composition, and the elimination of debris from the articular capsule. The main reason why the joint fails is insufficient lubrication due to disruption of the exchange between the bloodstream and the joint synovial shell inducing a deficit of the lubricating synovial fluid. The disruption of the exchange is compensated by injecting synovial fluid substitutes. The paper shows the results of tribological studies and clinical trials providing evidence that the patient’s blood serum can be used as an absolutely compatible lubricating fluid to restore the functions of the articular synovial medium.     

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.     

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.     

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.     

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.