Mass gain behaviour of tibial polyethylene inserts during soak testing

Fluid adsorption and the associated mass gain behaviour in tibial inserts of total knee replacements was investigated in polyethylene (PE) manufactured from extruded GUR 1050 resin. Repeatedly removing the PE inserts from the soak fluid for gravimetric assessment (including cleaning, desiccation, and weighing) increased the mass gain. Soaking PE inserts for 46 days or 92 days seemed to give about the same mass gain. PE inserts that were soaked at 37 degrees C gained more mass than PE inserts soaked at room-temperature. Gas-plasma sterilized PE inserts gained less mass than gamma-in-air sterilized PE inserts. No statistically significant differences were detected in mass gain between PE inserts that were of 10mm and 14mm thickness. The mass gain of PE inserts was higher in protein-rich soak fluid compared with low-ion distilled water. Prior to knee simulator wear testing, tibial PE inserts should be conditioned in the same medium and under the same test conditions (gravimetric assessment frequency, fluid protein content, and fluid temperature). This approach would help improve the accuracy and precision of the gravimetrically determined PE wear rate during knee simulator wear testing.     

Demonstration of a New Technique Using Radioisotope Tracers to Measure the Backside Wear Rate on Tibial Inserts

Local backside wear measurements on ultra-high molecular weight polyethylene (UHMWPE) tibial inserts in LCS mobile bearing knee prostheses have been performed using a new radioisotope tracing technique. The radioisotope tracers ⁹⁷Ru and ^101mRh were synthesized via a fusion evaporation reaction and recoil-implanted into cylindrical plugs of UHMWPE. The labelled plugs were carefully fitted into tibial inserts at two relevant locations. With bovine serum acting as a lubricant, the tibial inserts were then worn in vitro for 500,000 and 780,000 cycles, respectively, in a pneumatic knee motion simulator. Results reflect the non-linear change of wear during the wear-in phase and its evolution to a long-term steady-state rate. This new technique shows potential for extracting localized wear rates across the backside of a tibial insert in order to develop a comprehensive backside wear model.     

Long-term results for Kinemax and Kinematic knee bearings on a six-station knee wear simulator

A long-term wear test was performed on Kinemax and Kinematic (Howmedica Inc.) knee bearings on the Durham six-station knee wear simulator. The bearings were subjected to flexion/extension of 65-0 degrees, anterior-posterior translation of between 4.5 and 8.5 mm and a maximum axial load of 3 kN. Passive abduction/adduction and internal/external rotation were also permitted, however, two of the stations had a linkage system which produced +/- 5 degrees active internal/external rotation. The bearings were tested at 37 degrees C in a 30 per cent bovine serum solution and the test was run to 5.6 x 10(6) cycles. The bearings from stations 2 and 3, and stations 4 and 5 were swapped during the test to investigate the effects of interstation variability. The average wear rate and standard error was 3.00 +/- 0.98 mg/10(6) cycles (range 1.33-4.72 mg/10(6) cycles) for the Kinemax bearings and 3.78 +/- 1.04 mg/10(6) cycles (range 1.87-4.89 mg/10(6) cycles) for the Kinematic bearings. There were no significant differences in wear rates between the different bearing designs, the addition of active internal/external rotation or a change of stations. However, the wear tracks were different for the two types of bearings and with active internal/external rotation. The wear rates and factors were generally lower than previously published in vitro wear results; however, this may have been due to a difference in the axial loads and lubricants used. The appearance of the wear tracks with active internal/external rotation was comparable with those seen on explanted knee bearings.     

Deformation of the cement mantle of tibial components following total knee arthroplasty: a laboratory study.

An in vitro model has been developed to measure in-plane strains of the cement mantle, sandwiched between the tibial component and the underlying cancellous bone following total knee arthroplasty. Maximal in-plane strains occurred in the cement mantle below the contact points between the femoral and tibial components. These strains were significantly reduced by increasing the thickness of the polyethylene and even more impressively by metal backing. Eccentric loading, by as little as 5 degrees, increased the strains in the loaded compartment by 26 per cent and decreased those in the unloaded compartment by 62 per cent. The addition of torsion to axial loading did not significantly alter the principal direct strains or the principal shear strains. Although surface-covering tibial components have been advocated, continuous support of the cortical rim did not appear to be important in reducing cement mantle strains. While other studies have emphasized the critical stresses that may occur in the polyethylene tibial components of total knee implants, this study highlights the potential for localized cement fatigue with improperly sized components or with eccentric loading.     

The influence of femoral condylar lift-off on the wear of artificial knee joints

In vivo fluoroscopic studies of patients with total knee replacements (TKRs) have shown lift-off of the femoral condyles from the tibial insert. This study investigated the influence of femoral condylar lift-off on the ultra-high molecular weight polyethylene (UHMWPE) wear of fixed bearing (FB) and rotating platform mobile bearing (RP MB) total knee replacements, using a physiological knee joint simulator. In the absence of lift-off, the RP MB knees exhibited a lower wear rate of 5.2 +/- 2.2 mm3 per million cycles (mm3/MC) compared with 8.8 +/- 4.8 mm3/MC for the FB knees. The presence of femoral condylar lift-off was found to accelerate the wear of the FB and RP MB knees tested in this study to 16.4 +/- 2.9 and 16.9 +/- 2.9 mm3/MC respectively. For the RP MB knees the increase in wear rate was more marked, resulting in a similar wear rate for both designs of knee under lift-off conditions. In both cases the medial condyle displayed more wear damage. This study has shown that a small amount of abduction/adduction lift-off and medial-lateral shift increases wear and that the increase in wear is design dependent. In this simulator test, lift-off was simulated on every cycle, whereas the amount of wear and effect of lift-off clinically would depend on the frequency of occurrence of lift-off in vivo.     

Investigation of wear of knee prostheses in a new displacement/force-controlled simulator

The performance of two knee simulators designed by ProSim (Manchester, UK) was evaluated by comparison of the wear seen in the press-fit condylar (PFC) Sigma (DePuy) knee prosthesis. Twelve specimens of the same design and manufacturing specification, were subjected to a wear test of 2 x 10(6) cycles duration using bovine serum as a lubricant. The anterior/posterior displacement and internal/external rotation inputs were based on the kinematics of the natural knee. International Standards Organization (ISO) standards were used for the flexion and axial load. The wear rates and wear scar areas were compared across all stations. The mean wear rates found were 17.6+/-5 mm3/10(6) cycles for stations 1 to 6 and 19.6+/-4 mm3/10(6) cycles for stations 7 to 12, resulting in an overall mean wear rate of 18.1+/-3 mm3/10(6) cycles. The differences between the two simulators were not significant. The average wear scar area seen on inserts from stations I to 6 was calculated at 32.4+/-1 per cent of the intended articulating surface. Similarly on stations 7 to 12 the average wear scar area was 30.7+/-3 per cent. The wear scars seen were a good physiological representation of those found from clinical explant data. This study has shown good repeatability from the simulator, both within and between the simulators.     

Elasto-plastic contact analysis of an ultra-high molecular weight polyethylene tibial component based on geometrical measurement from a retrieved knee prosthesis

The wear phenomenon of ultra-high molecular weight polyethylene (UHMWPE) in knee and hip prostheses is one of the major restriction factors on the longevity of these implants. Especially in retrieved knee prostheses with anatomical design, the predominant types of wear on UHMWPE tibial components are delamination and pitting. These fatigue wear patterns of UHMWPE are believed to result from repeated plastic deformation owing to high contact stresses. In this study, the elasto-plastic contact analysis of the UHWMPE tibial insert, based on geometrical measurement for retrieved knee prosthesis, was performed using the finite element method (FEM) to investigate the plastic deformation behaviour in the UHMWPE tibial component. The results suggest that the maximum plastic strain below the surface is closely related to subsurface crack initiation and delamination of the retrieved UHMWPE tibial component. The worn surface whose macroscopic geometrical congruity had been improved due to wear after joint replacement showed lower contact stress at macroscopic level.     

Prediction of backside micromotion in total knee replacements by finite element simulation

The micromotion at the interface between the polyethylene tibial insert and metal tibial tray [corrected] in modular total knee replacements [corrected] has been shown to contribute to wear particle-induced osteolysis and may [corrected] cause implant failure. Therefore, studying the design parameters that are involved in the backside wear process is an important task that may lead to improvement in new total knee replacements. In the present study, a finite element model was developed to predict the backside micromotion along the entire modular interface. Both the linear elastic constitutive model and non-linear J2-plasticity constitutive model were considered in the finite element model for polyethylene and were corroborated against published results obtained from displacement controlled knee simulator wear tests. The finite element simulation with the non-linear J2-plasticity constitutive model was able to predict backside micromotion [corrected] more accurately than the simulation with the linear elastic constitutive model. [corrected] The developed finite element model (including the non-linear J2-plasticity constitutive model) was then applied to assess the effects of the tibial tray locking mechanism design (dovetails versus fullperipheral [corrected] design) and different levels of interference fit on insert micromotion. The developed finite element model, implementing the non-linear J2-plasticity constitutive model, was shown to successfully predict clinical amounts of backside micromotion and could be used for the design and development of total knee replacements for the reduction of backside micromotion and polyethylene [corrected] wear.     

The wear of fixed and mobile bearing unicompartmental knee replacements

Unicompartmental knee replacements (UKR) are an option for surgical intervention for the treatment of single-compartment osteoarthritis. The aim of this study was to compare the wear of a low-conformity fixed-bearing UKR with a conforming mobile bearing UKR under two kinematic conditions, to investigate the effect of implant design and kinematics on wear performance in a physiological knee wear simulator. Under both sets of kinematic conditions, the relatively low-conforming fixed UKR showed lower wear, compared with the more conforming anterior-posterior sliding mobile bearing. However, it should be noted that differences in materials between the two designs also contribute to the relative wear performance of the bearings. The combined wear of the medial and lateral bearings of the fixed-bearing UKR as a 'total knee' were significantly reduced compared with a fixed-bearing total knee replacement studied under the same kinematic conditions.     

A machine for the preliminary investigation of design features influencing the wear behaviour of knee prostheses

Degradation of tibial inserts in vivo has been found to be multifactorial in nature, resulting in a complex interaction of many variables. A range of kinematic conditions occurs at the tibio-femoral interface, giving rise to various degrees of rolling and sliding at this interface. The movement of the tibio-femoral contact point may be an influential factor in the overall wear of ultra-high molecular weight polyethylene (UHMWPE) tibial components. As part of this study a three-station wear-test machine was designed and built to investigate the influence of rolling and sliding on the wear behaviour of specific design aspects of contemporary knee prostheses. Using the machine, it is possible to monitor the effect of various slide roll ratios on the performance of contemporary bearing designs from a geometrical and materials perspective.     

Fluid composition impacts standardized testing protocols in ultrahigh molecular weight polyethylene knee wear testing

Wear of total knee replacements is determined gravimetrically in simulator studies. A mix of bovine serum, distilled water, and additives is intended to replicate the lubrication conditions in vivo. Weight gain due to fluid absorption during testing is corrected using a load soak station. In this study, three sets of ultrahigh molecular weight polyethylene tibial plateau were tested against highly polished titanium condyles. Test 1 was performed in two different institutions on the same simulator according to the standard ISO 14243-1, using two testing lubricants. Test 2 and test 3 repeated both previous test sections. The wear and load soak rates changed significantly with the lubricant. The wear rate decreased from 16.9 to 7.9 mg weight loss per million cycles when switching from fluid A to fluid B. The weight gain of the load soak specimen submersed in fluid A was 6.1 mg after 5 x 10(6) cycles, compared with 31.6 mg for the implant in fluid B after the same time period. Both lubricants were mixed in accordance with ISO 14243 (Implants for surgery - wear of total knee-joint prostheses), suggesting that calf serum should be diluted to 25 +/- 2 per cent with deionized water and a protein mass concentration of not less than 17 g/l. The main differences were the type and amount of additives that chemically stabilize the lubricant throughout the test. The results suggest that wear rates can only be compared if exactly the same testing conditions are applied. An agreement on detailed lubricant specifications is desirable.     

A five-station hip joint simulator

A five-station hip joint wear simulator was designed and built which featured simplified motion and loading. An elliptical wear path was produced using approximately sinusoidal motion in the flexion/extension and internal/external rotation axes and the dynamic loading approximated to a square wave. Five 28 mm diameter zirconia femoral heads articulated against ultra-high molecular weight polyethylene acetabular cups in 25 per cent bovine serum for 5 x 10(6) cycles. Gravimetric wear measurement was used with moisture absorption compensation using a dynamically loaded soak control. With motion of physiological magnitude, the mean acetabular cup wear rate was 52.2 mm3/10(6) cycles which is comparable with a number of clinical studies.     

An in vitro study of the reduction in wear of metal-on-metal hip prostheses using surface-engineered femoral heads

Although the wear of existing metal-on-metal (MOM) hip prostheses (1 mm3/10(6) cycles) is much lower than the more widely used polyethylene-on-metal bearings, there are concerns about the toxicity of metal wear particles and elevated metal ion levels, both locally and systemically, in the human body. The aim of this study was to investigate the possibility of reducing the volume of wear, the concentration of metal debris and the level of metal ion release through using surface-engineered femoral heads. Three thick (8-12 microm) coatings (TiN, CrN and CrCN) and one thin (2 microm) coating (diamond-like carbon, DLC), were evaluated on the femoral heads when articulating against high carbon content cobalt-chromium alloy acetabular inserts (HC CoCrMo) and compared with a clinically used MOM cobalt-chromium alloy bearing couple using a physiological anatomical hip joint simulator (Leeds Mark II). This study showed that CrN, CrCN and DLC coatings produced substantially lower wear volumes for both the coated femoral heads and the HC CoCrMo inserts. The TiN coating itself had little wear, but it caused relatively high wear of the HC CoCrMo inserts compared with the other coatings. The majority of the wear debris for all half-coated couples comprised small, 30 nm or less, CoCrMo metal particles. The Co, Cr and Mo ion concentrations released from the bearing couples of CrN-, CrCN- and DLC-coated heads articulating against HC CoCrMo inserts were at least 7 times lower than those released from the clinical MOM prostheses. These surface-engineered femoral heads articulating on HC CoCrMo acetabular inserts produced significantly lower wear volumes and rates, and hence lower volumetric concentrations of wear particles, compared with the clinical MOM prosthesis. The substantially lower ion concentration released by these surface-engineered components provides important evidence to support the clinical application of this technology.     

Nanoindentation properties of compression-moulded ultra-high molecular weight polyethylene

This paper investigates the elastic modulus and hardness of untreated and treated compression-moulded ultra-high molecular weight polyethylene (UHMWPE) tibial inserts of a total knee replacement (TKR) prosthesis. Investigations were carried out at a nanoscale using a Nanoindenter at penetration depths of 100,250 and 500 nm. The nanomechanical properties of surface and subsurface layers of the compression-moulded tibial inserts were studied using the untreated UHMWPE. The nanomechanical properties of intermediate and core layers of the compression-moulded tibial insert were studied using the cryoultrasectioned and etched UHMWPE treated samples. The cryoultrasectioning temperature (-150 degrees C) of the samples was below the glass transition temperature, Tg (-122 +/- 2 degrees C ), of UHMWPE. The measurement of the mechanical response of crystalline regions within the nanostructure of UHMWPE was accomplished by removing the amorphous regions using a time-varying permanganic-etching technique. The percentage crystallinity of UHMWPE was measured using differential scanning calorimetry (DSC) and the Tg of UHMWPE was determined by dynamic mechanical analysis (DMA). Atomic force microscopy (AFM) was used to assess the effect of surface preparation on the samples average surface roughness, Ra. In this study, it was demonstrated that the untreated UHMWPE samples had a significantly lower (p < 0.0001) elastic modulus and hardness relative to treated UHMWPE cryoultrasectioned and etched samples at all penetration depths. No significant difference (p > 0.05) in elastic modulus and hardness between the cryoultrasectioned and etched samples was observed. These results suggest that the surface nanomechanical response of an UHMWPE insert in a total joint replacement (TJR) prosthesis is significantly lower compared with the bulk of the material. Additionally, it was concluded that the nanomechanical response of material with higher percentage crystallinity (67 per cent) was predominantly determined by the crystalline regions within the semi-crystalline UHMWPE nanostructure.     

Thermal Behavior of a Worn Tilting Pad Journal Bearing: Thermohydrodynamic Analysis and Pad Temperature Measurement

Abstract

Tilting pad journal bearing (TPJBs) have been widely used in rotating machinery such as compressors and turbines due to their superior stability compared to that of conventional fixed geometry bearings. As the adoption of TPJBs increases, various failure mechanisms related to TPJBs have been reported and pad wear is a frequently reported one. Pad wear causes geometry changes of the bearing, which can sometimes result in entire system failure. Therefore, it is important to detect the problem in the early stage to prevent unexpected system shut down. The objective of the current research is to investigate the influence of pad wear on the pad temperature, which is one of the widely used condition monitoring methods of TPJBs. For the theoretical investigation, thermohydrodynamic (THD) analysis was conducted by solving the generalized Reynolds equation and 3D energy equation. The developed THD model was validated by comparing the results with test data available in the literature. The results of the analysis showed that the temperature of the loaded pad increases and that of the unloaded pad decreases when wear occurred on loaded pads. In addition, the minimum film thickness also decreases with the wear depth. The validation test conducted with a test rig which mimics axial turbine where a test rotor was supported by two TPJBs. The test bearing consists of five pads with diameter of 60 mm and a resistance temperature detector (RTD) was installed in the pad for temperature monitoring. The test was performed by replacing the two loaded pads with the worn pad. The test result for TPJB with wear depth of 30 μm showed that the temperatures of the loaded pads were 8°C higher and that of the unloaded pad was 2.5°C lower than that of the normal TPJB. Additionally, the predicted pad temperature shows good agreement with the measured pad temperatures.     

A study on wear and wear mechanism of exhaust valve and seat insert depending on different speeds using a simulator

The wear of engine valve and seat insert is one of the most important factors which affect engine performance. Because of higher demands on performance and the increasing use of alternative fuel, engine valve and seat insert are challenged with greater wear problems than in the past. In order to solve the above problems, a simulator was developed to be able to generate and control high temperatures and various speeds during motion. The wear simulator is considered to be a valid simulation of the engine valve and seat insert wear process with various speeds during engine activity. This work focuses on the different degrees of wear at three different singular test speeds (10 Hz, 25 Hz & multi-Hz). For this study, the temperature of the outer surface of the seat insert was controlled at 350°C, and the test load was 1960 N. The test cycle number was 6.0×10⁶. The mean (±standard error) wear depth of the valve at 10 Hz and 25 Hz was 45.1 (±3.7)μm and 81.7 (±2.5)μm, respectively. The mean wear depth of the seat insert at 10 Hz and 25 Hz was 52.7 (±3.9) μm and 91.2 (±2.7) μm, respectively. In the case of multi-Hz it was 70.7 (±2.4)/on and 77.4 (±3.8) μm, respectively. It was found that higher speed (25 Hz) cause a greater degree of wear than lower speed (10 Hz) under identical test condition (temperature, valve displacement, cycle number and test load). In the wear mechanisms of valves, adhesive wear, shear strain and abrasive wear could be observed. Also, in the wear mechanisms of seat inserts, adhesive wear, surface fatigue wear and abrasive wear could be observed.     

Influence of simulator kinematics on the wear of metal-on-metal hip prostheses

There is now considerable interest in metal-on-metal bearings for hip prostheses. Extremely low wear rates (0.1 mm3/10(6) cycles) have been reported in some simulator studies, while in vivo studies, although still very low, have shown wear rates of the order of 1 mm3/10(6) cycles. The aim of this study was to compare wear rates of metal-on-metal bearings in two hip simulators with different kinematic inputs. In the simulator with three independent input motions which produced an open elliptical wear path with a low level of eccentricity, the wear rates were very low as recorded previously in other simulators. In the simulator with two input motions which produced an open elliptical wear path with greater eccentricity the wear rate was at least ten times higher and closer to clinical values. The motion and kinematic conditions in the contact are critical determinants of wear in metal-on-metal bearings.     

Metal-on-metal hip simulator study of increased wear particle surface area due to 'severe' patient activity

This study investigated changes in metal-on-metal (MOM) hip wear and wear particle characteristics arising from a more aggressive patient activity level compared with normal walking. The test hypothesis was that 'severe'-gait conditions will change wear, wear particle sizes, and morphology owing to a decline in joint lubrication. Four carbon MOM hip bearings 40 mm high were subjected to normal-walking and fast-jogging simulations in an orbital hip joint simulator with 25 per cent alpha-calf serum as a lubricant. Co-Cr-Mo wear particles were extracted using an enzymatic method, and prolate ellipsoid equations were used to estimate particle volume and surface area. Fast-jogging simulations generated a sevenfold increase in volumetric wear, a 33 per cent increase in mean wear particle size, and a threefold increase in the number of larger (needle) particles compared with walking. This resulted in a twentyfold increase in total wear particle surface area per 10(6) cycles compared with walking, thereby confirming our hypothesis. The clinical significance of this result suggests that highly active MOM patients may exhibit greater ion release than less active patients.     

Quantification of third body damage to the tibial counterface in mobile bearing knees

Fourteen pairs of explanted low contact stress (LCS) tibial interface components: six rotating platform (RP), six meniscal (MN) and two anterior-posterior (AP) glide designs, have been analysed with particular attention paid to the condition of the tibial counterfaces. The average surface roughness, Ra, for the tibial trays ranged from 0.01 to 0.087 micron, significantly greater than the unworn control measurement of 0.008 micron. The scratch geometry analysis showed that the scratch peaks were found to be consistently of a lower aspect ratio than the scratch valleys and under 1 micron in height (average asperity height Rp = 0.52 micron, aspect ratio delta p = 0.01, average asperity depth Rv = 1.10 microns, delta v = 0.05). The largest scratches were 3-4 microns in both Rp and Rv. In vitro tests have shown that ultra-high molecular weight polyethylene (UHMWPE) wear increases in the presence of counterface scratches perpendicular to the direction of motion. In these explants, the unidirectional motion produced scratches parallel to the direction of sliding which is predicted to produce a smaller increase in UHMWPE wear. Other designs in mobile bearing knees have less constrained motion at the tibial counterface and this has been shown to accelerate wear; it may also lead to a further increase in wear in the presence of third body scratches. It may be possible in future knee designs to reduce this type of wear damage by introducing alternative materials or coatings which are more resistant to scratching and surface roughening.     

Wear behaviors of single-layer and multi-layer coated carbide inserts in high speed machining of hardened AISI 4340 steel

Flank wear progression and wear mechanisms of uncoated, coated with PVD applied single-layer TiAlN, and CVD applied multi-layer MT-TiCN/Al₂O₃/TiN cemented carbide inserts were analyzed during dry turning of hardened AISI 4340 steel (35 HRC). Experimental observations indicate that by applying a coating to the uncoated insert the limiting cutting speed increase from 62 to 200 m/min, which further extends up-to 300–350 m/min when using multi-layer coating scheme. Relatively lower wear rate seen when using single-layer TiAlN coated inserts. However, after removal of the thin layer of coating the wear rate increase rapidly, subsequently dominates the wear rate of multi-layer coated inserts. Cutting forces; especially axial and radial components have also shown the similar behavior and increase rapidly when the tool failure occurs. Flank wear, crater wear and catastrophic failure are the dominant forms of tool wear. Digital microscope and SEM images coupled with elemental analysis (EDAX) have been taken at various stages of tool life for understanding the wear mechanisms.