Competitive Binding of Bilirubin and Fatty Acid on Serum Albumin Affects Wear of UHMWPE

Total Joint Replacement (TJR) devices undergo standardized wear testing in mechanical simulators while submerged in a proteinaceous testing solution to mimic the environmental conditions of artificial joints in the human body. Typically, bovine calf serum is used to provide the required protein content. However, due to lot-to-lot variability, an undesirable variance in testing outcome is observed. Based on an earlier finding that yellowish-orange serum color saturation is associated with wear rate, we examined potential sources of this variability, by running a comparative wear test with bilirubin; hemin; and a fatty acid, oleic acid, in the lubricant. All these compounds readily bind to albumin, the most abundant protein in bovine serum. Ultrahigh molecular weight polyethylene (UHMWPE) pins were articulated against CoCrMo discs in a pin-on-disc tribometer, and the UHMWPE wear rates were compared between lubricants. We found that the addition of bilirubin increased wear by 121%, while hemin had a much weaker, insignificant effect. When added at the same molar ratio as bilirubin, the fatty acid tended to reduce wear. Additionally, there was a significant interaction with respect to bilirubin and hemin in that UHMWPE wear rate decreased with increasing fatty acid concentration. We believe the conformational change in albumin by binding bilirubin makes it more likely to form molecular bridges between UHMWPE and the metal counterface, thus increasing adhesive wear. However, fatty acids compete for binding sites on albumin, and can prevent this conformational change. Hence, the protein is stabilized, and the chance for albumin to form bridges is lowered. Ultimately, UHMWPE wear rate is driven by the competitive binding of bilirubin and fatty acid to albumin.

Sensitivity of total knee replacement wear to variability in motion and load input: A parametric finite element analysis study

Polyethylene wear remains a contributor to long term failure in total knee replacements (TKRs). Advances in materials have improved polyethylene wear rates, therefore further wear reductions require a better understanding of patient-specific factors that lead to wear. Variability of gait within patients is considerable and could lead to significant variability in wear rates that cannot be predicted by standard testing methods. An in-silico study was performed to investigate the influence of gait variability on TKR polyethylene wear. Nine characteristic peaks within the load and motion profiles used for TKR wear testing were varied 75% to 125% from baseline (ISO-14243-3:2014) to generate 310 unique waveforms. Wear was calculated for 1-million cycles using a finite element TKR wear model. From the results, a surrogate model was developed using multiple linear regression, and used to predict how wear changes due to dispersion of motion and force peaks within a) ±5%, the maximum allowable input tolerance of ISO, and b) ±25%, more reflective of patient gait inter-variability. The range of wear within the ±5% tolerance was 0.65 mm³ /million cycles and was 3.24 mm³ /million cycles within the ±25% variability more in line with the dispersion observed within patients. Although no one kinematic or kinetic peak dominated variability in TKR volumetric wear, variability within flexion/extension peaks were the largest contributor to wear rate variability. Interaction between the peaks of different waveforms was also important. This study, and future studies incorporating patient-specific data, could help to explain the connection between patient-specific gait factors and wear rates.

Structural and tribological characteristics of ultra-low-wear polyethylene as artificial joint materials

Ultra-low-wear polyethylene (ULWPE) is a new metallocene catalyzed high density polyethylene (HDPE)material. Previous studies have demonstrated that it has excellent biocompatibility and wear resistance, whereupon indicating great potential in the applications to artificial joints. However, as a newly developed material, its tribological behavior and wear resistance mechanism has not been well understood. In the current study, we experimentally evaluated the tribological behavior of ULWPE, and investigated its high wear resistance mechanism in terms of microstructure, crystallization properties, mechanical, physical, and chemical properties. ULWPE manifested the best tribological performance on pin-on-disc (POD) wear tests compared with the most widely used artificial joints materials, with a wear volume of 0.720 ± 0.032 mm³/million cycles (Mc) and 0.600 ± 0.027 mm³/Mc against cobalt-chromium (CoCr) alloy disc and zirconia toughened alumina (ZTA) ceramic disc, respectively. The results of the wear morphology analysis showed that the surface of ULWPE was the slightest, with no obvious surface damage, debris shedding and wear pits. We reveal that three major factors mainly contributed to its high wear resistance. First, ULWPE demonstrated a high crystallinity and a compact crystalline morphology comprised of long linear molecular chains, which contributed to its good mechanical performance. As confirmed by the mechanical test, ULWPE had a very high density, hardness, and tensile elongation at break. The high hardness and strength laid a solid foundation to a low wear volume, and its high ductility and hardness helped to endure abrasive and adhesive wear, resulting in excellent wear resistance. Second, the results of wettability analysis showed that the contact angle formed on the surface of ULWPE was the lowest and the surface energy was the highest. The hydrophilicity of ULWPE provided good lubrication conditions in body fluid. Third, it also had a lower oxidation index. The high hardness, high strength, high ductility and good wetting of ULWPE materials reduced the damage of the material to adhesion and abrasive wear, resulting in excellent wear resistance.

The choice of the femoral center of rotation affects material loss in total knee replacement wear testing - A parametric finite element study of ISO 14243-3

A leading cause of long-term failure of total knee replacements (TKRs) is osteolysis caused by polyethylene wear particles. The current gold standard for preclinical wear testing of TKRs is mechanical knee simulators. The definition of the femoral center of flexion-extension rotation (CoR) has been identified as one possible source of variability within TKR wear tests, since the femoral curvature varies from distal to posterior. The magnitude of the influence on wear due to changes in location of femoral CoR has not been investigated in depth. During this study, a computational framework utilizing finite element analysis for modelling wear of TKRs was developed and used to investigate the influence of the location of femoral CoR on TKR polyethylene wear during standardized displacement controlled testing (ISO 14243-3:2014). The study was carried out using a 40-point Latin Hypercube Design of Experiments approach. Volumetric wear was highly correlated to femoral CoR in both the superior/inferior and anterior/posterior directions, with a stronger relationship in the superior/inferior direction. In addition, wear scars showing linear penetration were examined, with large differences in simulations at the extreme ends of the sampling region. In this study, it was found that variations in the location of the femoral center of rotation can represent a large source of variability in the preclinical testing and evaluation of the wear performance of total knee replacements. This study represents the first attempt at quantifying the effect on wear of different femoral center of rotations across a large sampling space.

A comparison between electromechanical and pneumatic-controlled knee simulators for the investigation of wear of total knee replacements

More robust preclinical experimental wear simulation methods are required in order to simulate a wider range of activities, observed in different patient populations such as younger more active patients, as well as to fully meet and be capable of going well beyond the existing requirements of the relevant international standards. A new six-station electromechanically driven simulator (Simulation Solutions, UK) with five fully independently controlled axes of articulation for each station, capable of replicating deep knee bending as well as other adverse conditions, which can be operated in either force or displacement control with improved input kinematic following, has been developed to meet these requirements. This study investigated the wear of a fixed-bearing total knee replacement using this electromechanically driven fully independent knee simulator and compared it to previous data from a predominantly pneumatically controlled simulator in which each station was not fully independently controlled. In addition, the kinematic performance and the repeatability of the simulators have been investigated and compared to the international standard requirements. The wear rates from the electromechanical and pneumatic knee simulators were not significantly different, with wear rates of 2.6 ± 0.9 and 2.7 ± 0.9 mm³/million cycles (MC; mean ± 95% confidence interval, p = 0.99) and 5.4 ± 1.4 and 6.7 ± 1.5 mm³/MC (mean ± 95 confidence interval, p = 0.54) from the electromechanical and pneumatic simulators under intermediate levels (maximum 5 mm) and high levels (maximum 10 mm) of anterior-posterior displacements, respectively. However, the output kinematic profiles of the control system, which drive the motion of the simulator, followed the input kinematic profiles more closely on the electromechanical simulator than the pneumatic simulator. In addition, the electromechanical simulator was capable of following kinematic and loading input cycles within the tolerances of the international standard requirements (ISO 14243-3). The new-generation electromechanical knee simulator with fully independent control has the potential to be used for a much wider range of kinematic conditions, including high-flexion and other severe conditions, due to its improved capability and performance in comparison to the previously used pneumatic-controlled simulators.

Evidence for the dissolution of molybdenum during tribocorrosion of CoCrMo hip implants in the presence of serum protein

We have characterized CoCrMo, Metal-on-Metal (MoM) implant, wear debris particles and their dissolution following cycling in a hip simulator, and have related the results to the tribocorrosion of synthetic wear debris produced by milling CoCrMo powders in solutions representative of environments in the human body. Importantly, we have employed a modified ICP-MS sample preparation procedure to measure the release of ions from CoCrMo alloys during wear simulation in different media; this involved use of nano-porous ultrafilters which allowed complete separation of particles from free ions and complexes in solution. As a result, we present a new perspective on the release of metal ions and formation of metal complexes from CoCrMo implants. The new methodology enables the mass balance of ions relative to complexes and particles during tribocorrosion in hip simulators to be determined. A much higher release of molybdenum ions relative to cobalt and chromium has been measured. The molybdenum dissolution was enhanced by the presence of bovine serum albumin (BSA), possibly due to the formation of metal-protein complexes. Overall, we believe that the results could have significant implications for the analysis and interpretation of metal ion levels in fluids extracted from hip arthroplasty patients; we suggest that metal levels, including molybdenum, be analysed in these fluids using the protocol described here.

STATEMENT OF SIGNIFICANCE

We have developed an important new protocol for the analysis of metal ion levels in fluids extracted from hip implant patients and also hip simulators. Using this procedure, we present a new perspective on the release of metal ions from CoCrMo alloy implants, revealing significantly lower levels of metal ion release during tribocorrosion in hip simulators than previously thought, combined with the release of much higher percentages of molybdenum ions relative to cobalt and chromium. This work is of relevance, both from the perspective of the fundamental science and study of metal-protein interactions, enabling understanding of the ongoing problem associated with the biotribocorrosion and the link to inflammation associated with Metal-on-Metal (MoM) hip implants made from CoCrMo alloys.

Wear and creep behavior of total knee implants undergoing wear testing

We sought to determine what dimensional changes occurred from wear testing of a total knee implant, as well as any changes within the polyethylene subsurface. Three fixed bearing implants underwent wear simulator testing to 6.1 million cycles. Gravimetric analysis and micro-CT scans were performed pre-test, mid-test, and post-test. Wear volume and surface deviations were greater during 0-3.2 million cycles (91 ± 12mm(3)) than from 3.2 to 6.1 million cycles (52 ± 18mm(3)). Deviations (wear and creep) occurred across all surfaces of the tibial inserts, including the articular surface, backside surface, sides, and locking mechanism. No subsurface changes were found. The micro-CT results were a useful adjunct to gravimetric analysis, defining the dimensional changes that occurred with testing and ruling out subsurface fatigue.

CR TKA UHMWPE wear tested after artificial aging of the vitamin E treated gliding component by simulating daily patient activities

The wear behaviour of total knee arthroplasty (TKA) is dominated by two wear mechanisms: the abrasive wear and the delamination of the gliding components, where the second is strongly linked to aging processes and stress concentration in the material. The addition of vitamin E to the bulk material is a potential way to reduce the aging processes. This study evaluates the wear behaviour and delamination susceptibility of the gliding components of a vitamin E blended, ultra-high molecular weight polyethylene (UHMWPE) cruciate retaining (CR) total knee arthroplasty. Daily activities such as level walking, ascending and descending stairs, bending of the knee, and sitting and rising from a chair were simulated with a data set received from an instrumented knee prosthesis. After 5 million test cycles no structural failure of the gliding components was observed. The wear rate was with 5.62 ± 0.53 mg/million cycles falling within the limit of previous reports for established wear test methods.

How do gait frequency and serum-replacement interval affect polyethylene wear in knee-wear simulator tests?

Polyethylene wear (PE) is known to be a limiting factor in total joint replacements. However, a standardized wear test (e.g. ISO standard) can only replicate the complex in vivo loading condition in a simplified form. In this study, two different parameters were analyzed: (a) Bovine serum, as a substitute for synovial fluid, is typically replaced every 500,000 cycles. However, a continuous regeneration takes place in vivo. How does serum-replacement interval affect the wear rate of total knee replacements? (b) Patients with an artificial joint show reduced gait frequencies compared to standardized testing. What is the influence of a reduced frequency? Three knee wear tests were run: (a) reference test (ISO), (b) testing with a shortened lubricant replacement interval, (c) testing with reduced frequency. The wear behavior was determined based on gravimetric measurements and wear particle analysis. The results showed that the reduced test frequency only had a small effect on wear behavior. Testing with 1 Hz frequency is therefore a valid method for wear testing. However, testing with a shortened replacement interval nearly doubled the wear rate. Wear particle analysis revealed only small differences in wear particle size between the different tests. Wear particles were not linearly released within one replacement interval. The ISO standard should be revised to address the marked effects of lubricant replacement interval on wear rate.

Wear testing of crosslinked polyethylene: wear rate variability and microbial contamination

The wear performance of two types of crosslinked polyethylene (Marathon™ and XLK™, DePuy Synthes Inc., Warsaw, IN) was evaluated in a pin-on-disc wear tester, a hip wear simulator, and a knee wear simulator. Sodium azide was used as the microbial inhibitor in the calf serum-based lubricant. In the pin-on-disc wear tester, the Marathon wear rate of 5.33±0.54mm(3)/Mc was significantly lower (p=0.002) than the wear rate of 6.43±0.60mm(3)/Mc for XLK. Inversely, the Marathon wear rate of 15.07±1.03mm(3)/Mc from the hip wear simulator was 2.2-times greater than the XLK wear rate of 6.71±1.03mm(3)/Mc from the knee wear simulator. Differences in implant design, conformity, GUR type, and kinematic test conditions were suggested to account for the difference between the wear rates generated in the different types of wear testing apparati. In all wear tests, sodium azide was ineffective at inhibiting microbial growth in the lubricant. Eight different organisms were identified in the lubricant samples from the wear tests, which suggested the necessity of using an alternative, more effective microbial inhibitor. Careful sample preparation and thorough cleaning has shown to improve the consistency of the wear results. The wear rates generated in the hip and knee wear simulators closely reflected the wear behaviour of Marathon and XLK reported in published data that were tested under similar conditions.

Biochemical analyses of human osteoarthritic and periprosthetic synovial fluid

Biochemical analyses were performed on osteoarthritic and periprosthetic synovial fluid in order to propose changes to lubricant specifications currently outlined in orthopaedic wear testing standards. Osteoarthritic and periprosthetic synovial fluid samples were obtained from the hip and knee joints of 40 patients. The samples in each group were analysed and compared in order to identify differences between the protein concentration, constituent fractions, osmolality, thermal stability and the hyaluronic acid concentration and molecular weight distribution of osteoarthritic and periprosthetic synovial fluid. The average total protein concentration was approximately 30 g/L, which was much higher than the 20 g/L currently specified in the knee wear testing standard; however, the 30 g/L protein concentration matched the recently revised standard for hip simulator wear testing. No significant difference was found between the protein concentration, osmolality, thermal stability, and hyaluronic acid concentration of osteoarthritic and periprosthetic synovial fluid. The clinical data provided should be used to better define the composition of a more clinically relevant lubricant for orthopaedic wear testing.

Calf serum constituent fractions influence polyethylene wear and microbial growth in knee simulator testing

Calf serum lubricants consisting of various polypeptide constituent fractions are routinely used in knee wear simulators as part of the standardized test protocol. Three calf sera (bovine, new-born and alpha) were diluted as per the recommendation of ISO 14243-3 and used in displacement-controlled knee wear simulators to investigate their effects on polyethylene wear. Biochemical analyses included measuring total polypeptide degradation, electrophoretic profiles and low-molecular weight polypeptide concentrations to elucidate their involvement in the wear process. The effects of the various calf sera constituent fractions on microbial growth were also explored. The polyethylene wear rates and the results from the biochemical analyses for the three calf serum lubricants were all found to be statistically significantly different from each other. The lubricant derived from the alpha-calf serum was closest in constituent fractions to human synovial fluid. It also showed the lowest polyethylene wear rate (14.38 ± 0.85 mm3/million cycles) and the lowest amount of polypeptide degradation (7.77 ± 3.87%). Furthermore, the alpha-calf serum lubricant was associated with the least amount of change in the electrophoretic profile, the least change in low-molecular weight polypeptide concentration, and the lowest microbial growth in the presence of sodium azide (a microbial inhibitor conventionally used in implant wear testing). Replacing sodium azide with a broad spectrum antibiotic-antimycotic eradicated the microbial growth. Some speculation was entertained regarding the effect of alpha-calf serum on colloid-mediated boundary lubrication. Based on the results, it was recommended that ISO 14243-3 be modified to include guidelines on calf serum constituent fractions that would favour using alpha-calf serum in order to improve the fidelity of the simulation in knee implant wear testing.