Serum Lubricant Research Articles

The Effects of Implant Temperature on Lubricant Protein Precipitation and Polyethylene Wear in Joint Simulation Studies

Abstract Implant temperature increases as soon as a wear test starts. Frictional heating is the main reason for the temperature increase. Factors that affect the magnitude of the increase include the thermal conductivity of the implant materials, testing speed, and serum volume of the test chamber. High implant temperature can cause precipitation of soluble proteins in the serum lubricant. The protein precipitates can form a film at the wear interface and may affect laboratory wear evaluation of polyethylene liners. The effects of the implant temperature (of cobalt-chromium and zirconia femoral components) and the amount of protein precipitation from serum lubricant on the wear of UHMWPE liners has been demonstrated in a previous study using an external temperature control device. Furthermore, the nonlinear relationship of the implant temperature and the polyethylene wear has been discussed in another study, using moderately crosslinked UHMWPE liners tested against cobalt-chromium, zirconia, and alumina/zirconia composite femoral components at various temperatures. The results from both studies demonstrated that the temperature around the hip joint implants during a simulation can affect the wear rate of polyethylene. One guideline for minimizing the thermal effects on in vitro testing include circulation of coolant at an appropriate temperature to avoid overheating (due to nonstop running of the simulator) thus preventing excessive protein precipitation. This recommendation should help producing wear surfaces and wear debris that are more comparable to in vivo outcomes.

Increased Total Knee Arthroplasty Ultra-High Molecular Weight Polyethylene Wear Using a Clinically Relevant Hyaluronic Acid Simulator Lubricant

In this study, osteoarthritic and periprosthetic synovial fluid samples were rheologically and biochemically compared to develop a hyaluronic acid (HA) supplemented bovine serum (BS) lubricant that mimicked the properties of human joint synovial fluid. The effect of this BS + HA lubricant (50 per cent bovine calf serum + 1.5 g/l HA) on the wear rate of ultra-high molecular weight polyethylene (UHMWPE) during a total knee replacement wear test was then investigated. In conjunction with biochemical similarities, the rheological analysis showed that the BS + HA lubricant viscosity was not statistically different to aspirated total knee arthroplasty (TKA) revision joint fluid viscosity over a range of physiologic shear rates. Gravimetric results at 5 million wear testing cycles showed that the BS + HA lubricant produced an average of 6.88 times more UHMWPE wear than 50 per cent bovine serum lubricant alone. The BS + HA lubricated CoCr femoral component surfaces revealed pitting and surface roughening that was not observed using standard bovine serum only lubricants, but that was similar to the metallic surface corrosion observed on in vivo CoCr femoral component retrievals. These findings support the hypothesis that the addition of HA to simulator lubricant is capable of producing CoCr femoral component surface damage similar to that observed in vivo.

A Review of Lubrication Conditions for Wear Simulation in Artificial Hip Replacements

It is essential to evaluate the wear behavior of joint replacement materials and designs. While synovial fluid is the biological joint lubricant, it is present in too small quantities to use for wear simulations. A variety of lubricants have been used, such as water, saline solutions, synovial fluid components (hyaluronic acid, lubricin, and phospholipids), serum, and custom-made lubricants. We review studies from our Loma Linda University Medical Center lab and other labs using metal-on-polyethylene, exploring the benefits and deficiencies of these pseudo-synovial fluids. Serum is used extensively as the lubricant because its results most closely match clinical results with polyethylene bearings. We summarize laboratory experiments that have explored the effects of serum with differences in proteins (concentration, type, and ratio), volume, viscosity, temperature, and additives and their wear response. Protein precipitates seem to protect the implants from wear. The properties of current lubricants, in particular the roles of proteins, are the current focus in the search for an ideal joint analogue fluid. Presented at the ASME/STLE Tribology Conference in Long Beach, California October 24-27, 2004 Review led by Thierry Blanchet

Effect of Lubricant Composition on the Fatigue Properties of Ultra-High Molecular Weight Polyethylene for Total Knee Replacement

Ultrahigh molecular weight polyethylene (UHMWPE) fatigue is a critical factor affecting the longevity of total knee replacement (TKR) bearings. With the increased need for laboratory studies to mimic near in vivo conditions for accurate characterization of material performance, the present study investigated the role of hyaluronic acid (HA) in testing lubricant on the crack growth response of UHMWPE. It was hypothesized that the change in lubricant viscosity as a result of HA would affect the fatigue life of the polymer. A fracture mechanics approach as per ASTM E 647 was adopted for this study. Surface micrograph and surface chemistry analyses were employed to study the micromechanisms of fatigue failure and protein adsorption of the specimen surfaces. Rheological analysis indicated that the addition of HA to diluted bovine serum increased testing lubricant viscosity. HA concentrations of 2.22, 0.55, and 1.5 g/l closely matched the viscosity ranges reported for osteoarthritis, rheumatoid arthritic diseased joint fluid, and periprosthetic fluids respectively. Results showed that the addition of HA to standard diluted bovine serum lubricants, in concentrations similar to that of periprosthetic fluid, delayed crack initiation and crack growth during fatigue testing.

The effect of microstructure on the wear of cobalt-based alloys used in metal-on-metal hip implants

The influence of microstructure on the wear of cobalt-based alloys used in metal-on-metal hip implants was investigated in a boundary lubrication regime designed to represent the conditions that occurred some of the time in vivo. These cobalt-chromium-molybdenum alloys were either wrought, with a total carbon content of 0.05 or 0.23 wt %, cast with a solution-annealing procedure or simply as-cast but not solution annealed. Bars of these different alloy grades were subjected to various heat treatments to develop different microstructures. The wear was evaluated in a linear-tracking reciprocating pin-on-plate apparatus with a 25 per cent bovine serum lubricant. The wear was found to be strongly affected by the dissolved carbon content of the alloys and mostly independent of grain size or the carbide characteristics. The increased carbon in solid solution caused reductions in volumetric wear because carbon helped to stabilize a face-centred cubic crystal structure, thus limiting the amount of strain-induced transformation to a hexagonal close-packed crystal structure. Based on the observed surface twining in and around the contact zone and the potentially detrimental effect of the hexagonal close-packed phase, it was postulated that the wear of cobalt-based alloys in the present study was controlled by a deformation mechanism.

A 12-Station Anatomic Hip Joint Simulator

A novel 12-station hip joint simulator with an anatomic position of the prosthesis was designed and built. The motion of the simulator consists of flexion-extension and abduction-adduction. The load is of the double-peak type. The validation test was done with three similar 28 mm CoCr-polyethylene joints in diluted calf serum lubricant for 3.3 x 10(6) cycles. The bearing surfaces of the polyethylene cups were burnished, the CoCr heads were undamaged, the wear particles were in the 0.1-1 microm size range, and the mean wear factor of the polyethylene cups was 5.7 x 10(-7) mm(3)/N m. These essential observations were in good agreement with clinical findings. In addition, three similar 50 mm CoCr/CoCr joints, representing the contemporary large-diameter metal-on-metal articulation were tested. The wear of the CoCr/CoCr joints was calculated from the Co and Cr concentrations of the used lubricant quantified with atomic absorption spectroscopy. The bearing surfaces of the CoCr/CoCr joints showed mild criss-cross scratching only. The average wear factor of polyethylene cups was 275 times that of the CoCr/CoCr joints. The tribological behaviour of the large-dia. CoCr/CoCr appeared to be dominated by fluid film lubrication, as indicated by very low frictional heating and wear, making it tribologically superior to the conventional CoCr/polyethylene, and therefore very interesting clinically. In conclusion, the simulator proved to be a valid, reliable, practical, economical, and easy-to-operate tool for wear studies of various hip replacement designs.

Hip simulator wear comparison of metal-on-metal, ceramic-on-ceramic and crosslinked UHMWPE bearings

Alumina ceramic-on-crosslinked polyethylene (C-CLPE), metal-on-metal (M/M) and alumina-on-alumina ceramic (C/C) articulations are high performance, low wear options for hip replacement bearings. Our objective was to compare the hip joint simulator wear performance of these alternative bearing couples to each other and to a control bearing of standard ultra-high molecular weight polyethylene (UHMWPE). Contemporary hip joint simulators were used for wear testing. All components were tested at 1 Hz under a joint reaction profile (2450 N maximum) and 50% diluted alpha-calf serum lubrication. Standard lubricant replacement, specimen cleaning and wear loss measurement methods were followed. Crosslinked polyethylene (PE) showed a 10-fold reduction in wear over control UHMWPE liners, regardless of head size. M/M components showed a nine-fold reduction in wear over the control PE. Wear performance of the M/M bearings was non-linear and highly variable; one M/M sample had a wear rate similar to that of the PE controls. Wear rates for C-CLPE and C/C were far less variable than for M/M. C-CLPE bearing wear was more consistent than M/M, although both had similar average wear. C/C showed a consistent 500-fold wear reduction over control PE and a 50-fold reduction over M/M or C-CLPE.

Alumina hip joints characterized by run-in wear and steady-state wear to 14 million cycles in hip-simulator model.

The biphasic wear performance (run-in; steady-state phase) of 28-mm alumina-alumina hip implants was studied by hip simulator methods using bovine serum as the lubricant. The Biolox implants were run to 5.7 million cycles and Bioceram implants to 14.4 million cycles (Mc). Wear with all-alumina total hip replacements (THR) first showed a high wear rate of the order 1.2 mm3/Mc, lasting approximately 0.17 Mc. Overall to 0.7 Mc, the run-in phase appeared curvilinear but could be described by a linear phase averaging 0.3 mm3/Mc. From 0.7 to 1 Mc duration, the wear trend transitioned into a steady-state phase. Wear rates from 1 to 14 Mc were of the order 0.02 mm3/Mc. Surface contamination from the serum lubricant resulted in cyclic weight fluctuations of the order 0.2 mg. The transition from average run-in to steady-state phase represented a wear reduction of 13-fold. Comparing the steady-state wear value to that in standard 28-mm UHMWPE CUPS approaching 75 mm3/year, there was clearly a three-orders-of-magnitude wear superiority in favor of ceramic cups.

Comparison of in vitro with in vivo characteristics of wear particles from metal-metal hip implants.

The purpose of the present study was to compare wear particles isolated from metal-metal (MM) hip implants worn in an orbital bearing simulator with particles from similar MM total hip replacement (THR) implants worn in vivo. Comparison of these particles is important because it will help to assess the overall suitability of this type of hip simulator for reproducing in vivo wear and for producing physiological wear particles suitable for biological studies of in vitro cellular response. Commercial grade components made of ASTM F75 (cast) alloy were evaluated. Simulator tests were performed in 95% bovine calf serum with a 28-mm-diameter implant. Wear particles were collected from 0 to 0.25 million cycles (run-in wear period) and 1.75 to 2 million cycles (steady-state wear period). Tissues from seven patients with MM implants (surface replacement or stem type) were harvested at revision surgeries (after 1-43 months). Metal wear particles were isolated from serum lubricant or tissues using an enzymatic protocol that was previously optimized to minimize particle changes due to reagents. After isolation, particles were centrifuged, embedded in epoxy resin, and characterized by transmission electron microscopy (TEM) and energy dispersive X-ray analysis (EDXA). Results of EDXA on particles from the hip simulator primarily indicated a predominance of particles containing Cr and O but no Co (most likely chromium oxide particles), and fewer CoCrMo particles presenting varying ratios of Co and Cr. Image analysis of TEM micrographs demonstrated that the majority of the particles from the simulator were round to oval, but a substantial number of needle-shaped particles were also found, especially from 0 to 0.25 Mc. The particles generated from 0 to 0.25 Mc had an average length of 53 nm, whereas those generated from 1.75 to 2 Mc had an average length of 43 nm. In vivo, EDXA and TEM analysis of particles that were retrieved from two patients at 23 and 43 months respectively, revealed that they were the most comparable in composition, average length (57 nm), and shape to particles generated in the hip simulator during the run-in wear period. Because a large clinical retrieval study in the literature suggested that a run-in wear regime might occur in vivo for some 6-36 months, the fidelity of the simulator of the present study was strongly supported. However, some uncertainties existed, including the finding that the particles isolated from the other five patients generated from 1 month up to 15 months (shorter implantation times than the other two patients) were smaller and mostly contained only Cr and O (no Co). In the opinion of the authors, this particular very short term patient group was somewhat atypical. Therefore, despite these uncertainties, the present study was deemed to support the ability of the orbital bearing hip simulator to produce physiological wear particles.

Wear of retrieved UHMWPE hip liners.

After the gamma-irradiation sterilization, the most widely used orthopaedic grade polymer bearing liner material for the total joint replacement, ultra-high molecular weight polyethylene (UHMWPE), degrades through the progressive in vivo oxidation. The oxidative degradation makes UHMWPE brittle and leads to reduction of its mechanical properties. In this study, the effect of the in vivo post-irradiation ageing time on the wear of UHMWPE was investigated. Twelve retrieved polyethylene hip liners implanted for 3-16 years and then stored in the air for 1.5-8 years were used. Two types of the pin-on-disk wear testing were conducted. The uni-directional repeat pass rotating and the linear reciprocating wear testing were done with stainless steel disks against stationary polyethylene pins under 4MPa at 1Hz with bovine serum lubrication. Wear of the retrieved polyethylene hip liners does not have significant correlation with the in vivo or total ageing time. The linear reciprocal sliding motion generated a more pronounced wear than the uni-directional repeat pass sliding motion. This indicates that the kinematic motion significantly affects the wear of aged UHMWPE, having a brittle, white band region.

The effects of lubricant composition on in vitro wear testing of polymeric acetabular components.

Lubricant composition is known to affect in vitro (simulator) wear of polymeric acetabular components. Clinical acetabular components, fabricated from both ultrahigh molecular weight polyethylene (UHMWPE) and polytetrafluoroethylene (PTFE), were tested against cobalt-chromium 32-mm ball heads, with the use of various lubricants, with different compositions and concentrations of bovine and calf serum as well as hyaluronic acid, in tests which lasted one million cycles. The type of proteins and their relative concentration in the lubricant affected the wear rate of both UHMWPE and PTFE. Increasing protein concentration reduced the UHMWPE wear rate but increased the PTFE wear rate. For a given total protein concentration, increasing the albumin/globulin (A/G) ratio led to a significant reduction in the wear rate for PTFE. A similar but smaller effect was found for UHMWPE. The relative wear-rate ratio between PTFE and UHMWPE depends on both the total protein concentration and the specific protein concentration (albumin/globulin ratio). The average clinical wear rates for both PTFE and UHMWPE and the average wear-rate ratios between them were reproduced when the serum lubricant contained total protein concentration and A/G ratio within normally observed physiological ranges. It is recommended that lubricants for simulator testing be standardized to control total protein content as well as albumin/globulin ratio.

Wear rate and surface morphology of UHMWPE cups are affected by the serum lubricant concentration in a hip simulation test

The wear rate and surface morphology of both cross-linked and non-cross-linked UHMWPE cups against cobalt–chromium alloy balls were evaluated when lubricated with bovine serum diluted to 90 or 25% of its original content in a laboratory hip simulation. The wear rates for the cross-linked polyethylene (PE) lubricated with 90 and 25% bovine serum were 20.4±0.1 and 34.1±2.6 (mg/million cycles), respectively. The wear rates for the non-cross-linked PE lubricated with 90 and 25% bovine serum were 43.9±1.8 and 57.8±0.6 (mg/million cycles), respectively. Similar morphology, i.e. in the form of generally smooth surfaces with many fine scratches, was observed under light microscopy for both materials lubricated with 90% serum. In contrast, roughened and irregular regions were on the articular surface of the non-cross-linked PE cups lubricated with 25% serum after 1 million cycles and remained throughout the remainder of the test. The cross-linked PE tested with 25% serum did not develop the surface appearance until 3.5 million wear cycles. For comparison purposes, PE cups that were revised due to PE wear were also examined. The cup surfaces appeared generally smooth and covered with scratches. However, no irregular surface was found as in our simulated cups lubricated with 25% serum. The results from this study support that cross-linked PE has better wear resistance than non-cross-linked PE. Serum lubricant with higher protein concentration provides better lubrication and caution must be taken when using serum with lower protein concentration, as it may create non-clinically relevant wear morphology.

The performance of gamma- and EtO-sterilised UHMWPE acetabular cups tested under severe simulator conditions. Part 2: wear particle characteristics with isolation protocols

Ultra-high molecular-weight polyethylene (UHMWPE) has been used in total joint replacement for the last three decades. Despite the recent advancements in prosthesis design, the wear of UHMWPE remains a serious clinical problem; the release of wear debris may induce osteolysis and implant loosening. Controlling the quality of the polyethylene is essential to improve its wear resistance and any potential adverse effect caused by processing, manufacturing or sterilisation should be avoided. To evaluate the influence of the sterilisation method (gamma-irradiation and ethylene oxide (EtO)-treatment) and third-body particles, gamma- and EtO-sterilised UHMWPE acetabular cups were tested against CoCrMo femoral heads in a hip joint simulator run for 2.5 million cycles in bovine calf serum in the presence of third-body polymethylmethacrylate (PMMA) particles. A method not requiring ultra-centrifugation has been proposed for the isolation of polyethylene wear debris from the serum lubricant. SEM analysis allowed debris shape and morphology to be determined, and the wear mechanism operating in this study to be hypothesised. The morphological features of the wear debris were in agreement with clinical findings, enabling the hip simulator function to be validated. Micro-Raman spectroscopy coupled to PLS analysis showed that the mechanical friction during in vitro tests induced significant crystallinity changes in all the cups. The most significant changes were observed for the EtO-sterilised cups, which showed the highest weight loss.

The effect of frictional heating and forced cooling on the serum lubricant and wear of UHMW polyethylene cups against cobalt–chromium and zirconia balls

Hip simulator tests of femoral balls of cobalt–chromium alloy or zirconia against acetabular cups of UHMW polyethylene were run with and without a coolant circulated inside the femoral balls. Without cooling, the wear of polyethylene against zirconia was about 48% lower than with cobalt–chromium alloy, but the steady-state temperature of the zirconia ball was higher (55°C vs. 41°C), and there was more precipitation of protein from the serum, which sometimes formed an adherent layer on the surface of the zirconia. Circulating coolant at 1–20°C markedly reduced the bearing temperatures and the protein precipitation. With coolant at 4°C, wear of the polyethylene against cobalt–chromium alloy was about 26% lower than against zirconia, but the macroscopic and microscopic appearance of the worn polyethylene surfaces were unlike that typically generated in vivo. With or without coolant, the morphology of the polyethylene wear debris was comparable to that generated in vivo, but the ratio of fibrillar to granular debris was higher at the reduced temperature. These results suggested that circulating coolant at an appropriate temperature could avoid overheating (due to non-stop running of the simulator), preventing excessive protein precipitation while providing wear surfaces and wear debris with morphologies closely comparable to those generated in vivo.

The wear of PTFE against stainless steel in a multi-directional pin-on-plate wear device

The aim of this work was to see if a multi-directional pin-on-plate wear device was able to reproduce the wear factors reported for hip prostheses which employed polytetrafluoroethylene (PTFE) acetabular cups articulating against stainless steel counterfaces. The influence of type of motion and protein concentration of the bovine serum lubricant on wear, were investigated. Tests involved a four-station pin-on-plate rig, two stations had a standard reciprocation-only motion, while the other two stations had a rotational element added to the test pins. Tests were done at 100, 50, 25, 10 and 0% bovine serum, the remaining lubricant being made up with distilled water. One hundred percent bovine serum was equivalent to a protein content of 52 g/l. Reciprocation plus rotation applied to the test pins produced mean wear factors which increased with protein concentration, as reported for PTFE rubbing against a metal counterface in hip simulators. With reciprocation-only motion, this trend was not clear. At zero protein content, wear was negligible. With reciprocation plus rotation applied to the test pins, wear factors in the range (2.3–5.2)×10 −5 mm 3/Nm were measured, which compare with a calculated clinical wear factor of 3.7×10 −5 mm 3/Nm.

Effect of extent of motion and type of load on the wear of polyethylene in a biaxial hip simulator.

The most commonly used wear test device for prosthetic hip joints is the so-called biaxial rocking motion (BRM) design. The design has been criticized for its excessive sliding distance per cycle. The design was modified so that the extent of motion was reduced from 46 to 23 degrees, and comparative tests were run with the use of 1-kN static load. The present authors have earlier used 1-kN static load with good results. To further confirm this finding, additional, comparative tests were run with double-peak dynamic load and 46 degrees extent of motion. All three tests (46 degrees /static, 23 degrees /static and 46 degrees /dynamic), were done with conventional ultra-high-molecular-weight polyethylene acetabular cups against polished CoCr femoral heads in diluted calf serum lubricant. In all tests, the principal cup wear mechanism was adhesive, manifested as burnishing. With respect to wear particles, those produced in the 46 degrees /dynamic test showed the lowest resemblance to particles isolated from periprosthetic tissues. The 46 degrees /dynamic test produced a mean wear rate 43% higher than 46 degrees /static, whereas the 46 degrees /static and 23 degrees /static tests produced almost identical values. The results indicated that the 46 degrees extent of motion and dynamic load may not always be the optimal combination in BRM tests.

239 炭素強化材料の牛血清溶液中における摩耗特性

239 炭素強化材料の牛血清溶液中における摩耗特性

Wear Characteristics of Ultrahigh Molecular Weight Polyethylene (UHMWPE)

The wear of ultrahigh molecular weight polyethylene (UHMWPE) bearing against 316 stainless steel or cobalt chromium (Co-Cr) alloy was measured using a 12-channel wear tester especially developed for the evaluation of candidate materials for prosthetic joints. The coefficient of friction and wear rate were determined as a function of lubricant, contact stress, and metallic surface roughness in tests lasting 2–3 million cycles, the equivalent of several years use of a prosthesis. Wear was determined by the weight loss of the polyethylene (PE) specimens corrected for the effect of fluid absorption. The friction and wear processes in blood serum differed markedly from those in saline solution or distilled water. Only serum lubrication produced wear surfaces resembling those observed on removed prostheses. The experimental methods provided accurate reproducible measurement of PE wear. The long-term wear rates were proportional to load and sliding distance. Although the PE wear rate increased with increasing surface roughness, wear was not severe except with very coarse metal surfaces. The data obtained in these studies formed a comparison basis for the subsequent evaluation of potentially superior materials for prosthetic joints.

Effect of Frictional Heating on the Serum Lubricant and Wear of UHMWPE Cups against Cobalt-Chrome or Zirconia Balls

Effect of Frictional Heating on the Serum Lubricant and Wear of UHMWPE Cups against Cobalt-Chrome or Zirconia Balls

Effects of hip-loading input on simulated wear of Al2O3–PTFE materials

The objective of this study was to compare the effects of static, sinusoidal and physiological load-profiles on wear of Al2O3–PTFE materials. This was an accelerated wear model of clinical relevance. In nine experiments, the peak load-levels were varied from 1 to 4kN in a hip simulator with multi-directional kinematics and with bovine serum used as the lubricant. Systematic wear differences were checked using three sizes of femoral heads in each experiment. The Paul load-profile used was found to be more aggressive than sinusoidal, raising the polytetrafluoroethylene (PTFE) wear-rates by 28%. The PTFE cups showed a very mild response to increased load magnitudes, only 11–20% increase evident in volumetric wear per 1kN increase in load. One recommendation was that simulator wear-studies adopt a 0.25–2.5kN Paul load-profile as their standard. An experiment with 0.84kN constant-load also performed satisfactorily, with PTFE wear-rates actually higher than with the 1kN sine and Paul load-profiles. Some wear anomalies were encountered due to the use of serum lubrication. Combinations of large head size, high load-magnitudes, the Paul load-profile and the high serum protein concentrations used in this study were at times contributing factors. Use of low-protein serum solution may be advisable for wear studies, not only to properly simulate the polymeric wear characteristics but also to minimize the degradation artifacts more prevalent in higher protein-concentrations.