A. Unsworth

Comparison of friction and lubrication of different hip prostheses

Abstract 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.

A frictional study of total hip joint replacements.

Polymeric wear debris produced by articulation of the femoral head against the ultra-high-molecular-weight polyethylene socket of a total hip replacement has been implicated as the main cause of osteolysis and subsequent failure of these implants. Potential solutions to this problem are to employ hard bearing surface combinations such as metal-on-metal or ceramic-on-ceramic prostheses. The aim of this study was to investigate the difference in lubrication modes and friction of a range of material combinations using synthetic and biological fluids as the lubricants. 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 carboxy methyl cellulose (CMC) fluids as the lubricant. Under these conditions the ceramic-on-ceramic joints showed full fluid film lubrication while the metal-on-metal, metal-on-plastic, diamond-like carbon-coated stainless steel (DLC)-on-plastic and ceramic-on-plastic prostheses operated under a mixed lubrication regime. With bovine serum as the lubricant in the all ceramic joints, however, the full fluid film lubrication was inhibited due to adsorbed proteins. In the metal-on-metal joints this adsorbed protein layer acted to reduce the friction while in the ceramic coupling the friction was increased. The use of bovine serum as the lubricant also significantly increased the friction in both the metal-on-plastic and ceramic-on-plastic joints. The friction produced by the DLC-on-plastic joints depended on the quality of the coating. Those joints with a less consistent coating and therefore a higher surface roughness gave significantly higher friction than the smoother, more consistently coated heads.

The effects of lubrication in hip prostheses

Three groups of experiments are reported, each looking at a particular aspect of lubrication of artificial hip joints so that some indication of the modes of operation could be found. The first group investigates the difference between material combinations when lubricated with synovial fluid and when dry, and also compares joints of the same material, radius and general form but having very different apparent surface areas. This leads to useful data concerning hydrodynamic lubrication. The second group of experiments investigates the effects of lubricant viscosity on the frictional resistance of artificial joints and the third group looks at the effects of static and suddenly applied loads on lubricated prosthetic hip joints.

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

Abstract 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.

Some new evidence on human joint lubrication.

Theoretical consideration has been given to the use of pendulum machines which are used to examine the frictional properties of human joints by incorporating them as fulcra. As a result, a new type of pendulum machine has been built which incorporates the facility to apply sudden loads to the joint on starting the swinging motion, and also the ability to measure directly the frictional torque experienced by the joint. The results obtained from natural hip joints indicate the presence of squeeze film lubrication under conditions of sudden loading of a joint. In addition, a self-generated fluid film process was observed at low loads while at higher loads boundary lubrication appeared to be important. These results have been used to describe the lubrication regimens occurring in a normal activity such as walking. A single experiment carried out on a hip from a patient suffering from severe rheumatoid arthritis has also been reported and the frictional resistance was seen to be increased fifteenfold compared to a normal hip.

Friction in hip prostheses

Although the reduction of frictional torques was the driving force behind the design of the Charnley prosthesis, later concerns about wear and subsequent loosening of this and other hip replacements have dominated debate within the bioengineering community. To stimulate discussion on the role of friction in loosening, a review of the frictional characteristics of different prostheses was undertaken. The use of simple laboratory screening-type machines in the frictional assessment of different material combinations is discussed together with experiments performed on single axis simulators using both conventional and experimental prostheses. In particular, recent developments in the use of soft layer components are highlighted. Further, the possible link between excessively high frictional torques and loosening is discussed in the light of current results obtained from explanted prostheses.

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

Abstract 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 × 106 cycles and then a lower steady state wear phase. The steady state wear rate of the 22 μUm radial clearance metal-on-metal joint was lower than that for the 40 μUm 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).

Wear studies on the likely performance of CFR-PEEK/CoCrMo for use as artificial joint bearing materials

It is well known that a reduction in the volume of wear produced by articulating surfaces in artificial joints is likely to result in a lower incidence of failure due to wear particle induced osteolysis. Therefore, new materials have been introduced in an effort to produce bearing surfaces with lower, more biologically acceptable wear. Polyetheretherketone (PEEK-OPTIMA) has been successfully used in a number of implant applications due to its combination of mechanical strength and biocompatibility. Pin-on-plate wear tests were performed on various combinations of PEEK-OPTIMA and carbon fibre reinforced PEEK-OPTIMA (CFR-PEEK) against various CoCrMo alloys to assess the potential of this material combination for use in orthopaedic implants. The PEEK/low carbon CoCrMo produced the highest wear. CFR-PEEK against high carbon or low carbon CoCrMo provided low wear factors. Pin-on-plate tests performed on ultra-high molecular weight polyethylene (UHMWPE) against CoCrMo (using comparable test conditions) have shown similar or higher wear than that found for CFR-PEEK/CoCrMo. This study gives confidence in the likelihood of this material combination performing well in orthopaedic applications.

The effect of surface topography of retrieved femoral heads on the wear of UHMWPE sockets

The study was undertaken to investigate the association, if any, between the surface roughness of 35 explanted femoral heads and the clinical wear factors of the corresponding polyethylene sockets. The wear of the socket was measured using a shadowgraph technique. The surface topography was investigated using a Rodenstock RM 600 non-contacting profilometer, and six parameters were used to characterise the roughness. Further, qualitative inspections of the femoral surface were undertaken using both a Joel JSM-IC-848 scanning electron microscope and a Zeiss Axiotech microscope with a differential interference contrast facility. Two parameters were found to correlate with the clinical wear factor, namely the skewness of the amplitude distribution function, Sk, and the arithmetic mean roughness, Ra. Simple parameters describing the peak heights of the asperities were found not to have a significant association with the clinical wear factor. The exponent in the power relationship between the arithmetic mean roughness and kclinical was found to be equal to 0.5 (SE: 0.2). This value is significantly smaller than that found in laboratory experiments and may be due to the non-uniform nature of the roughening of the femoral head, three-body wear and the effect of other clinical factors on the wear process. Further, extremely strong correlations were detected between the differing roughness parameters, which would suggest that any attempt to deduce which one is the most important in affecting the wear of the polymeric counterface is fraught with difficulty. However, further investigation of those parameters, such as the reduced peak height or the material filled profile peak area, which may better describe the effect of the counterface on the wear of the UHMWPE surface would appear to be prudent.

Frictional properties of artificial hip joints

A new generation of hip replacements has been designed incorporating compliant layers to promote fluid film lubrication when the joints are implanted in patients. Tests in the Durham hip function simulator show that the friction in these joints is up to an order of magnitude lower than in currently used prostheses, and because this is due to complete separation of the rubbing surfaces, wear ought to be vastly reduced.Experiments have shown that the best results are achieved with compliant surfaces of hardness between 4 and 8 N/mm2. Such surfaces produce coefficients of friction of the order of 5 × 10-3.