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.     

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.     

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.     

Frictional resistance of new and explanted artificial hip joints

The frictional resistance of 54 explanted Charnley joints and 32 new joints (various types) has been measured in a hip function simulator in dry and lubricated environments.

The friction factors (torque/(normal load × radius) ) for new prostheses were remarkably similar no matter what material or head size was used provided the acetabular component was made of ultra high molecular weight polyethylene (UHMWPE).

Charnley joints, implanted for up to 17 years, were removed from the patients at revision operations and showed that most had a friction factor similar to that of new joints. However, 30% had a friction factor greater than 0.16 when dry and 39% showed friction factors in excess of 0.07 when lubricated (cf 0.1 and 0.04 respectively when new).

All the joints operated under a mixed lubrication regimen.

When the acetabular component was made from UHMWPE, the friction factors were hardly affected by the material of the femoral component. 30% of explanted joints had friction almost twice as great as new joints, but loosening of the prostheses also occurred in some joints whose friction remained low.     

A test procedure for artificial finger joints

This paper highlights the lack of an agreed testing standard for artificial finger joints. It reviews the anatomy, pathology and biomechanics of finger joints as well as the various designs of finger prostheses and the machines used to test them. While pre-implantation testing should be fundamental, increasing regulation of the biomedical engineering industry will further demand testing of prostheses to pre-agreed standards. Standards relating to the testing of other artificial joints are reviewed before possible parameters for testing finger prostheses are offered.     

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 and simulation of a cable-pulley-based transmission for artificial ankle joints

In this paper, a mechanical transmission based on cable pulley is proposed for human-like actuation in the artificial ankle joints of human-scale. The anatomy articular characteristics of the human ankle is discussed for proper biomimetic inspiration in designing an accurate, efficient, and robust motion control of artificial ankle joint devices. The design procedure is presented through the inclusion of conceptual considerations and design details for an interactive solution of the transmission system. A mechanical design is elaborated for the ankle joint angular with pitch motion. A multi-body dynamic simulation model is elaborated accordingly and evaluated numerically in the ADAMS environment. Results of the numerical simulations are discussed to evaluate the dynamic performance of the proposed design solution and to investigate the feasibility of the proposed design in future applications for humanoid robots.     

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.     

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.     

Tribology of polymers

Tribological studies were performed on the friction and wear behaviour of polymers under conditions of dry sliding. The investigations were carried out with thermoplastics suitable for practical applications, eg HDPE, PP, PTFE, PA 6, PA 66, POM, PETP, PBTP, PI, as well as with some filled and reinforced polymers and composite materials. For polymer-polymer sliding pairs, the experimentally determined friction values could be related to the surface energies of the material pairings. In the case of polymer-metal sliding pairs, a relationship between the combined interfacial stresses and the rupture strength of the polymers was found. In addition to the review of correlations between the tribological behaviour of thermoplastics and material properties, the dependency of wear and friction on surface roughness, sliding velocity and contact pressure for various filled polymers is described.     

摩擦学研究及发展趋势

简要评述了摩擦学研究的进展.指出极端条件下的摩擦、磨损与润滑性能,分子水平上的摩擦磨损本质,环境友好润滑材料,摩擦学失效问题及摩擦学设计是摩擦学研究的主要方向.     

Tribology of videotapes

Increased recording density in magnetic recording has been achieved by introducing newly developed magnetic tapes, such as metal particulate tapes and metal-evaporated tapes. As a loss in magnetic recording increases with spacing between a magnetic tape and a magnetic head, these are obliged to contact each other. A tape also rubs tape-transportation elements in videotape recorders (VTRs), such as the stationary drum and guide posts. Thus it is not too much to say that the development of new magnetic tapes for practical application depends on tribological research.

In this paper, tribological analyses for the improvements of durability and runnability of metal particulate tapes and metal-evaporated videotapes are reviewed.     

摩擦学的进展和未来

在过去的20年内,随着纳米技术的飞速发展和人们社会需求的日益增加,摩擦学迅速发展,并随之产生了几个新的领域,比如纳米摩擦、生物摩擦、超滑、表面织构摩擦学、极端工况摩擦学、微动摩擦学等等。在未来的10年,这些领域和其他新出现的概念,比如:绿色摩擦、纳米制造摩擦学、新型超滑材料和新能源领域摩擦学等等,将在摩擦学研究工作中发挥重要的作用。纳米摩擦学包括纳米尺度下的薄膜润滑、纳米摩擦、纳米磨损、表面黏附等等。绿色摩擦学包括环境友好润滑剂、摩擦噪声的减小、没有环境污染的磨损。生物摩擦学包括人类器官中的摩擦学和仿生摩擦学。超滑包含不同类型的润滑剂,比如类金刚石膜、水基润滑剂、一些生物润滑剂,其具有极低的摩擦因数(0.001量级)。纳米制造摩擦学包括纳米结构制造中的摩擦学、纳米精度制造中的摩擦学和跨尺度(微观、中观和宏观)制造中的摩擦学。这些领域需要不同的理论和技术,以适应不同的制造对象和尺度。极端工况摩擦学包括在重载下、在高/低温度下、在高/低速度下和强腐蚀或氧化环境条件下的摩擦学问题。与表面织构相关的摩擦学理论和技术也是当今发展较快的领域。这些领域的新进展和未来10年的新需求将在下文中具体介绍。     

Effect of motion inputs on the wear prediction of artificial hip joints

Hip joint simulators have been largely used to assess the wear performance of joint implants. Due to the complexity of joint movement, the motion mechanism adopted in simulators varies. The motion condition is particularly important for ultra-high molecular weight polyethylene (UHMWPE) since polyethylene wear can be substantially increased by the bearing cross-shear motion. Computational wear modelling has been improved recently for the conventional UHMWPE used in total hip joint replacements. A new polyethylene wear law is an explicit function of the contact area of the bearing and the sliding distance, and the effect of multidirectional motion on wear has been quantified by a factor, cross-shear ratio. In this study, the full simulated walking cycle condition based on a walking measurement and two simplified motions, including the ISO standard motion and a simplified ProSim hip simulator motion, were considered as the inputs for wear modelling based on the improved wear model. Both the full simulation and simplified motions generated the comparable multidirectional motion required to reproduce the physiological wear of the bearing in vivo. The predicted volumetric wear of the ProSim simulator motion and the ISO motion conditions for the walking cycle were 13% and 4% lower, respectively, than that of the measured walking condition. The maximum linear wear depths were almost the same, and the areas of the wear depth distribution were 13% and 7% lower for the ProSim simulator and the ISO condition, respectively, compared with that of the measured walking cycle motion condition.