Tribology of total artificial joints.

The tribology of total artificial replacement joints is reviewed. The majority of prosthesis currently implanted comprise a hard metallic component which articulates on ultra high molecular weight polyethylene surface. These relatively hard bearing surfaces operate with a mixed or boundary lubrication regime, which results in wear and wear debris from the ultra high molecular weight polyethylene surface. This debris can contribute to loosening and ultimate failure of the prostheses. The tribological performance of these joints has been considered and a number of factors which may contribute to increased wear rates have been identified. Cushion bearing surfaces consisting of low elastic modulus materials which can articulate with full fluid film lubrication are also described. These bearing surfaces have shown the potential for greatly reducing wear debris.     

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.     

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.     

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.     

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.     

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.     

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.     

MoS2/Ti复合固体润滑膜的力学性能与摩擦特性

采用非平衡磁控溅射法在9Cr18轴承钢基底上制备了厚度约3μm的MoS2/Ti复合固体润滑膜,基于球形压头纳米压痕试验,采用连续刚度法对MoS2/Ti复合固体润滑膜的力学性能进行研究,探究MoS2/Ti复合固体润滑膜力学性能随压痕深度的变化规律;根据压痕试验载荷-位移曲线,采用Hertz接触理论计算MoS2/Ti复合固体润滑膜的弹性模量并与试验结果进行对比;利用CSM摩擦试验机对低速、低载下MoS2/Ti复合固体润滑膜的摩擦特性进行研究;基于压痕试验提出了一种能够更准确计算钢球加载时MoS2/Ti复合固体润滑膜接触应力的方法,并计算了摩擦试验不同载荷下的接触应力。结果表明:MoS2/Ti复合固体润滑膜的力学性能和摩擦特性都会受到表面形貌的影响;除表面初始压入阶段外,MoS2/Ti复合固体润滑膜的弹性模量和接触刚度都随着压痕深度的增大而增大;滑动速度和载荷共同影响MoS2/Ti复合固体润滑膜的摩擦特性。     

A new formulation for the prediction of polyethylene wear in artificial hip joints

Artificial joints employing ultra-high molecular weight polyethylene (UHMWPE) are widely used to treat joint diseases and trauma. Wear of the polymer bearing surface largely limits the use of these joints in younger and more active patients. Previous studies have shown the wear factor used in Archard's law for the conventional polyethylene to be highly dependent on contact pressure and this has produced variability in experimental data and has constrained the reliability and applicability of previous computational predictions. A new wear law is proposed, based on wear volume being dependent on, and proportional to, the product of the sliding distance and contact area. The dimensionless proportional constant, wear coefficient, which was independent of contact pressure, was determined from a multi-directional pin on plate study. This was used in computational predictions of the wear of the conventional UHMWPE hip joints. The wear of the polyethylene cup was independently experimentally determined in physiological full hip joint simulator studies. The predicted wear rate from the new computational model was generally increased, with an improved agreement with the experimental measurement compared with the previous computational model. It was shown that wear in the UHMWPE hip joints increased as head size and contact area increased. This resulted in a much larger increase in the wear rate as the head size increased, compared with the previous computational model, and is consistent with clinical observations. This new understanding of the wear mechanism in artificial joints using the UHMWPE bearing surfaces, and the improved ability to predict wear independently and to address previously described discrepancies offer new opportunities to optimize design parameters.     

Adsorption control of synovia constituents on artificial joint materials by means of an electric field: evaluation of tribological characteristics

The supplemental lubrication mechanism by means of an electric field was proposed to reduce friction and wear for the advanced joint prosthesis with the low elastic modulus bearing surface. The possibility of application of this mechanism to the prosthesis was investigated by the fundamental and experimental procedures in simplified sliding conditions. Conductive silicon rubber was used as the low elastic modulus surface. The counterfaces were a titanium alloy and a stainless steel. Protein (gamma-globulin) in lubricant appeared to cause the tribological characteristic to deteriorate in the mixed lubrication regime. High friction seemed to be brought about by the obstruction against the entraining of the fluid flow and the high shear stress due to the microbonding between the asperities of bearing surfaces, which were derived from the adsorbed protein on the hydrophobic surfaces. The repulsive force between the adsorbed film and the bearing materials by means of the d.c. electric field, and the continuous change in polarity on the surface by means of the a.c. electric field appeared to affect the adsorbed film adjacent to the bearing material, so that friction and wear were varied.     

化学反应膜对齿面润滑状态影响的研究

通过对齿面微观接触以及流体膜承载能力和边界膜承载能力相互关系的分析，从化学反应膜形成速度与磨损速度的平衡关系入手，建立了齿面化学反应膜对齿面润滑状态影响的数学模型，并通过试验验证了这一模型。结果表明：合适的化学反应膜，使表面形貌向着有利于润滑方向发展，使边界润滑向弹性流体润滑状态转变，从而提高齿面的承载能力。     

弹性模量对水润滑轴承的性能影响

从材料的角度,研究弹性模量对水润滑塑料合金轴承的性能影响,包括轴承承载能力、水膜形状曲线、压力曲线和最小水膜厚度等。研究表明:在选择水润滑轴承材料时应根据其承载能力来确定轴承材料的弹性模量范围;在承载能力范围内,从弹性模量这个角度来说,应当尽可能选择弹性模量低并具有良好摩擦磨损性能的材料,因为选择较小弹性模量的材料有利于提高最小水膜厚度,从而改善水润滑轴承摩擦副的摩擦磨损性能。     

An axisymmetric contact model of ultra high molecular weight polyethylene cups against metallic femoral heads for artificial hip joint replacements.

Contact mechanics of ultra high molecular weight polyethylene (UHMWPE) cups against metallic femoral heads for artificial hip joints is considered in this study. Both the experimental measurement of the contact area and the finite element prediction of the contact radius, maximum contact pressure and maximum Von Mises stress have been carried out for a wide range of contemporary artificial hip joints. Good agreement of the contact radius has been found between the experimental measurements and the finite element predictions based upon an elastic modulus of 1000 MPa and a Poisson's ratio of 0.4 for UHMWPE material under various loads up to 2.5 kN. It has been shown that the half contact angle for all the cup/head combinations considered in this study is between 40 degrees and 50 degrees under a load of 2.5 kN. The importance of this result has been discussed with respect to the anatomical position of the cup when placed in the body and the selection of a simple wear-screening test for artificial hip joints. The predicted contact radius and maximum contact pressure from the finite element model have also been compared with a simple elasticity analysis. It has been shown that the difference in the predicted contact radius between the two methods is reduced for more conforming contacts between the femoral head and the acetabular cup and smaller UHMWPE cup thickness. However, good agreement of the predicted maximum contact pressure has been found for all the combinations of the femoral head and the acetabular cup considered in this study. The importance of contact mechanics on the clinical performance of artificial hip joint replacements has also been discussed.     

Elastohydrodynamic lubrication analysis of a functionally graded layered bearing surface,with particular reference to 'cushion form bearings' for artificial knee joints

Elastohydrodynamic lubrication of a functionally graded layered (FGL) bearing surface, whose elastic modulus increases with depth from the bearing surface, was investigated in this study. The finite difference method was employed to solve the Reynolds equation, simultaneously with the elasticity equation of the bearing surface, under circular point contacts. The finite element method was adopted to solve the elasticity equation for the FGL bearing surface. The displacement coefficients thus obtained were used to calculate the elastic deformation of the bearing surface, required for the elastohydrodynamic lubrication analysis. Good agreement of the predicted film thickness and pressure distribution was obtained, between the present method and a previous study for a single layered bearing surface with a uniform elastic modulus. The general numerical methodology was then applied to an FGL bearing surface with both linear and exponential variations in elastic modulus, with particular reference to the 'cushion form bearing' for artificial knee joints. The predicted film thickness and pressure distribution were shown to be quite close to those obtained for a single layer under typical operating conditions representative of artificial knee joints, provided that the elastic modulus of the single layer was chosen to be the average elastic modulus of the graded layer.     

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。     

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.     

Carbon coating of Ti-6Al-4V for reduced wear in combined impact and sliding applications

A range of carbon coatings with different hardness and modulus was compared for wear and frictional behaviours using one-side-carbon-coated Ti-6Al-4V alloy couples tested under conditions of combined impact and sliding contact. Carbon films with hardness over 10 GPa were found to cause far greater volume loss of the uncoated counterpart, and the volume loss was approximately proportional to the extent of hardness deviation above 10 GPa. The coefficient of friction was shown to correlate positively with coating hardness. The tendency of a softer coating to possess a greater sp² or graphite-like content provides more effective solid lubrication in a wet environment, hence minimising both wear and friction. The corresponding low film modulus also provides an optimal structural integrity of the composite system by minimising the elastic modulus mismatch between the film and the underlying substrate.     

Stress analysis of cushion form bearings for total hip replacements.

Cushion form bearings comprise a thin layer of low elastic modulus material on the articulating surface of the bearing, which can deform to help preserve a film of lubricant between the bearing surfaces and therefore reduce friction and wear. The long-term function of this type of bearing is dependent on the strength and durability of this compliant layer. Finite difference and finite element methods have been used to analyse the stress distribution in the compliant layer of cushion form bearing for artificial hip joints under physiological loading conditions. A good agreement between finite difference and finite element methods was found. Under normal loading, the highest value of the maximum shear stress was found to be at the interface between the compliant layer and the more rigid substrate close to the edge of the contact. The values of maximum shear stress in the centre of the contact close to the articulating surface were lower than in the equivalent Hertzian contact. A friction force acting at the surface had little effect on the stress distribution for coefficients of friction less than 0.05. However, for higher values of friction coefficient (larger than 0.2), corresponding to inadequate lubrication, the maximum shear stress increased by a factor of four and was found to be located at the surface. The analysis predicts that the mode of failure will be at the interface with the substrate under fluid film or mixed lubrication conditions and at the articulating surface when the bearing runs dry with higher levels of friction. Both failure modes have been observed experimentally under the conditions specified.     

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.     

In Situ Studies of Cartilage Microtribology: Roles of Speed and Contact Area

The progression of local cartilage surface damage toward early stage osteoarthritis (OA) likely depends on the severity of the damage and its impact on the local lubrication and stress distribution in the surrounding tissue. It is difficult to study the local responses using traditional methods; in situ microtribological methods are being pursued here as a means to elucidate the mechanical aspects of OA progression. While decades of research have been dedicated to the macrotribological properties of articular cartilage, the microscale response is unclear. An experimental study of healthy cartilage microtribology was undertaken to assess the physiological relevance of a microscale friction probe. Normal forces were on the order of 50 mN. Sliding speed varied from 0 to 5 mm/s, and two probes radii, 0.8 and 3.2 mm, were used in the study. In situ measurements of the indentation depth into the cartilage enabled calculations of contact area, effective elastic modulus, elastic and fluid normal force contributions, and the interfacial friction coefficient. This work resulted in the following findings: (1) at high sliding speed (V = 1–5 mm/s), the friction coefficient was low (μ = 0.025) and insensitive to probe radius (0.8–3.2 mm) despite the fourfold difference in the resulting contact areas; (2) the contact area was a strong function of the probe radius and sliding speed; (3) the friction coefficient was proportional to contact area when sliding speed varied from 0.05 to 5 mm/s; (4) the fluid load support was greater than 85% for all sliding conditions (0% fluid support when V = 0) and was insensitive to both probe radius and sliding speed. The findings were consistent with the adhesive theory of friction; as speed increased, increased effective hardness reduced the area of solid–solid contact which subsequently reduced the friction force. Where the severity of the sliding conditions dominates the wear and degradation of typical engineering tribomaterials, the results suggest that joint motion is actually beneficial for maintaining low matrix stresses, low contact areas, and effective lubrication for the fluid-saturated porous cartilage tissue. Further, the results demonstrated effective pressurization and lubrication beneath single asperity microscale contacts. With carefully designed experimental conditions, local friction probes can facilitate more fundamental studies of cartilage lubrication, friction and wear, and potentially add important insights into the mechanical mechanisms of OA.