Long-term wear of HIPed alumina on alumina bearings for THR under microseparation conditions

The long term wear and wear debris generated in HIPed alumina on alumina bearings for hip prostheses with microseparation in vitro is compared to standard simulator conditions and ex vivo specimens. Microseparation studies were completed to five million cycles at two severity levels in attempts to rigorously evaluate the long-term tribological performance of the bearings. During the first million cycles (bedding-in) of the microseparation tests characteristic stripe wear was observed on all of the femoral heads with a matching area on the rim of the acetabular inserts. Under mild microseparation conditions an average wear rate of 0.55 mm3/million cycles was observed during the initial million cycles which reduced to a steady state level of 0.1 mm3/million cycles. Under more severe conditions an average wear rate of 4.0 mm3/million cycles was observed during bedding-in which reduced to a steady state level of 1.3 mm3/million cycles. These compare to a bedding-in wear rate of 0.11 mm3/million cycles and steady-state wear rate of 0.05 mm3/million cycles for the same material under normal simulation with no microseparation. Furthermore, under microseparation the wear mechanisms and wear debris were similar to those observed in previous alumina retrieval studies with debris ranging from 10 nm to 1 microm in size.     

Effect of the design parameters on the in vitro wear performance of total shoulder arthroplasties

The loosening of the glenoid component is the main reason for the failure of a total shoulder arthoplasty. It may be caused either by high tensile stresses or by osteolysis of the surrounding bone in response to the presence of particle debris. This failure might be associated with the wear of the implant as occurs with replacement hip and knee joints.The paper reports the findings of a study of the in vitro performance of the currently used total shoulder prostheses to determine the effects of implant geometry on the wear of the polyethylene components and the friction conditions operating within the shoulder prosthesis.The wear performance of the implants was evaluated using a self-developed tribotester, simulating the physiological conditions of a shoulder joint. This study revealed that significantly different wear occurred with conforming and non-conforming articulation and revealed the influence of the thickness of the polyethylene glenoid on the wear and friction occurring in the joint. In this preliminary study significant wear of the polyethylene glenoid component occurred, estimated to be up to 19 mm³/year, which is similar to that found in retrieved implants. The conforming implants demonstrated significantly greater wear than the non-conforming implants (p < 0.05). A significantly lower friction factor, about 0.05 ± 0.01 (p < 0.05), was obtained for the less conforming implants.     

Wear of surface engineered metal-on-metal hip prostheses

The wear of existing metal-on-metal (MOM) hip prostheses (1 mm3/million cycles) is much lower than the more widely used polyethylene-on-metal bearings (30-100 mm3/million cycles). However, there remain some potential 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 wear, wear debris and ion release of fully coated surface engineered MOM bearings for hip prostheses. Using a physiological anatomical hip joint simulator, five different bearing systems involving three thick (8-12 microm) coatings, TiN, CrN and CrCN, and one thin (2 microm) coating diamond like carbon (DLC) were evaluated and compared to a clinically used MOM cobalt chrome alloy bearing couple. The overall wear rates of the surface engineered prostheses were at least 18-fold lower than the traditional MOM prostheses after 2 million cycles and 36-fold lower after 5 million cycles. Consequently, the volume of wear debris and the ion levels in the lubricants were substantially lower. These parameters were also much lower than in half coated (femoral heads only) systems that have been reported previously. The extremely low volume of wear debris and concentration of metal ions released by these surface engineered systems, especially with CrN and CrCN coatings, have considerable potential for the clinical application of this technology.     

The influence of surface topography on wear debris generation at the cement/bone interface under cyclic loading

The long-term success of a total joint replacement can be undermined by loosening of the implant, generation of wear debris or a combination of both factors. In the present study the influence of the surface morphologies of the bone and cement mantle on loosening of cemented total joint replacements (THJRs) and development of wear debris were studied. Model cemented THJR specimens were prepared in which the femoral canal was textured using specific cutting tools. The specimens were subjected to cyclic loads inducing pure shear fatigue of the cement/bone interface. Changes in both the femoral canal and cement mantle resulting from fatigue were quantified in terms of the surface topography and the volume of wear debris. Loosening occurred with cyclic loading due to degradation of the cement and bone and resulted in the development of cement and bone particles. There was no correlation between the fatigue strength of the interfaces and the volume of wear debris. In general, the change in surface topography of the cement mantle with fatigue decreased with increasing volume of cement interdigitation. Femoral canal surfaces with symmetric profile height distribution (i.e., Gaussian surfaces) resulted in the lowest volume of generated debris.     

Polyethylene wear particles induce TLR 2 upregulation in the synovial layer of mice

A cellular and receptor mediated response to ultra-high-molecular-weight-polyethylene (UHMWPE) wear particles results in a release of proinflammatory cytokines and induces an inflammatory reaction causing osteolysis in total joint replacement. This investigation offers insight into the toll-like receptor (TLR) mediated activation by polyethylene wear particles in the synovial layer of mice. We hypothesized that, similar to recent in vitro results, UHMWPE particles lead to an upregulation of TLR 1 and 2 and TLR 4 in vivo in the synovial tissue of mice as well. Therefore, UHMWPE particles were generated in a common knee simulator according to the ISO standard, separated by acid digestion and determined by scanning electron microscopy. Endotoxin was removed using a method based on ultracentrifugation. A particle suspension (50 μl; 0.1 vol./vol.%) was injected into the left knee joint of female Balb/c mice (n = 8). In a control group, phosphate-buffered saline was injected into the left knee of Balb/c mice (n = 8). The mice were sacrificed after 7 days. Immunohistochemical staining was performed with TLR 1, 2 and 4 polyclonal antibodies for Balb/c mice and evaluated by light microscopy. The particle-stimulated group showed a thickened synovial layer, an increased cellular infiltration and a TLR 2-upregulation in the synovial layer compared to the control group. An increased expression of TLR 1 and TLR 4 could not be demonstrated. These results indicate a mainly TLR 2-induced inflammation to polyethylene wear debris in the synovial layer of mice.     

The influence of acetabular cup angle on the wear of “BIOLOX Forte” alumina ceramic bearing couples in a hip joint simulator

The wear of Biolox Forte alumina ceramic bearing couples has been investigated at two different acetabular cup angles in a physiological hip joint simulator. All cups were set in the anatomical position of 45° inclination in the M/L plane for the first two million cycles and then four of the six cups were re-aligned to 60° for a further three million cycles. A running-in wear of 0.14 mm₃ per million cycles was observed for the first million cycles, after which a steady state wear rate of 0.05 mm₃ per million cycles was observed. Increasing the acetabular cup angle to 60° did not significantly affect the wear rate.     

Biological activity and migration of wear particles in the knee joint: an in vivo comparison of six different polyethylene materials

Wear of polyethylene causes loosening of joint prostheses because of the particle mediated activity of the host tissue. It was hypothesized that conventional and crosslinked polyethylene particles lead to similar biological effects around the knee joint in vivo as well as to a similar particle distribution in the surrounding tissues. To verify these hypotheses, particle suspensions of six different polyethylene materials were injected into knee joints of Balb/C mice and intravital microscopic, histological and immunohistochemical evaluations were done after 1 week. Whereas the biological effects on the synovial layer and the subchondral bone of femur and tibia were similar for all the polyethylenes, two crosslinked materials showed an elevated cytokine expression in the articular cartilage. Furthermore, the distribution of particles around the joint was dependent on the injected polyethylene material. Those crosslinked particles, which remained mainly in the joint space, showed an increased expression of TNF-alpha in articular cartilage. The data of this study support the use of crosslinked polyethylene in total knee arthroplasty. In contrast, the presence of certain crosslinked wear particles in the joint space can lead to an elevated inflammatory reaction in the remaining cartilage, which challenges the potential use of those crosslinked polyethylenes for unicondylar knee prostheses.     

Comparison of wear in a total knee replacement under different kinematic conditions

A six station ProSim (Manchester, UK) knee simulator was used to assess the wear of six PFC (DePuy) fixed bearing total knee replacements under two different kinematic conditions defined as low and high kinematic inputs. The high kinematics displacement and rotation inputs were based on the kinematics of the natural knee with ISO standards used for the axial load and flexion. Low kinematics were defined as approximately half the magnitude. The six specimens were run for three million cycles under low kinematics and three million cycles under high kinematics. The mean wear rate found during the low kinematics phase was 7.7 +/- 2 mm3 per million cycles. This then increased significantly to an average wear rate of 41 +/- 14 mm3 during the high kinematics input phase. The wear areas were characterized by a predominant damage mode of burnishing with some abrasive wear occurring during the high kinematics phase. This study supports the findings that introduction of cross-shearing of the polyethylene by introducing both rotational and anterior/posterior displacement increases the wear rate. This has implications for younger patients with higher levels of activity that need knee replacements.     

Mechanical characterization of anti-infectious,anti-allergic,and bioactive coatings on orthopedic implant surfaces

In total joint replacement much effort has been made to reduce implant loosening. We investigated different implant coatings (copper integrated titanium dioxide (TiO₂–Cu), titanium nitride (TiN), plasma polymerized allylamine (PPAAm), and calcium phosphate (CaP)) regarding the adhesion strength and wear resistance. Standardized scratch and adhesive tests were applied. Abrasive wear was measured with artificial bone and bone cement using a special testing machine. All tested coatings have higher bonding strengths than the 22 N/mm² required for medical implant surface coatings by ASTM standard 4711-F. Using bone cement, wear testing revealed higher wear rates in most cases. Polished surfaces reduce the amount of wear, whereas rough surfaces highly increase the wear rate due to three-body wear, especially ceramic surfaces. In general, the application of bone cement in conjunction with modified implant surfaces can lead to an increase in wear rate.     

In vitro wear assessments of fixed and mobile UHMWPE total knee replacement

This work discusses the wear behaviour of two different ultra-high-molecular-weight-polyethylene tibial component designs. Mobile and fixed bearings were tested on a knee wear simulator for 5 million cycles using bovine calf serum as lubricant. We correlated the wear results with the chemical characterisation of the investigated materials: Fourier Transformed Infra Red Spectroscopy analyses, Differential Scanning Calorimetry and cross-link density measurements were used to assess the chemical features of this polyethylene.Mobile and fixed polyethylene inserts showed a different wear behaviour: the mobile designs components showed lower weight losses than the fixed components (109 ± 6 mg and 163 ± 80 mg, respectively). Significant statistical differences were observed in wear rate (P = 0.035, Kolmogorov–Smirnov Test for two samples).From a molecular point of view, typical radiation-induced oxidation profiles were observed in all the tested polyethylene samples, but the overall degradation was more significant in the fixed bearing inserts and this is likely to play a role on the wear performances.     

Quantitative analysis of polyethylene wear debris, wear rate and head damage in retrieved Charnley hip prostheses

Submicrometer- and micrometer-sized ultra-high molecular weight polyethylene (UHMWPE) wear particles have been associated with osteolysis and failure of total artificial joints. Previous studies have isolated predominantly submicrometer-sized particles at the expense of larger particles (>10 m). This study aimed to isolate and characterize quantitatively all sizes of UHMWPE wear particles generated in 18 Charnley hip prostheses. In addition, to analyze the wear debris with respect to the total volumetric wear of the cup and damage to the femoral head. Particle size distributions ranged from 0.1 to ->1000 m. A significant proportion (3–82%) of the mass of the wear debris isolated was>10 m. The mode of the frequency distribution of the particles was in the range 0.1–0.5 m for all patients. However, analysis of the mass of wear debris as a function of its size allowed differentiation of the wear debris from different patients. Femoral head damage was associated with high volumetric wear and increased numbers of biologically active submicrometer-sized particles.     

Evaluation of the friction and wear properties of PTFE composites filled with glass and carbon fiber

The aim of the research article is to study the mechanical and two‐body abrasive wear behaviour of glass/carbon fiber reinforced PTFE composites. The measured wear volume loss increases with increase in abrading distance. The results showed that the highest specific wear rate is for glass fiber reinforced PTFE composite with a value of 8.1×10^–6 mm³/Nm and the lowest wear rate is for carbon fabric reinforced vinyl ester composite with a value of 7.2×10^–6 mm³/Nm. Mechanical properties were evaluated and obtained values are compared with the wear behaviour. The worn surface features have been examined using scanning electron microscope (SEM). Photomicrographs of the worn surfaces revealed higher percentage of broken glass fiber as compared to carbon fiber.     

An evaluation of the tribological performance of zirconia and CoCrMo femoral heads

Five new zirconia, five new CoCrMo and three explanted CoCrMo femoral heads were wear-tested in bovine serum for five million cycles using the Durham Hip Joint Wear Simulator. Wear was measured gravimetrically and surface topography with a 3D non-contacting profilometer. This allowed an evaluation of the different head types on UHMWPE acetabular cup wear rates and the effect of roughening of the femoral head on acetabular cup wear. The mean acetabular cup wear rate against the five CoCrMo femoral heads was 40.8 mm³/10⁶ cycles which was significantly higher (p = 0.03) than against zirconia (33.3 mm³/10⁶ cycles). The initial surface roughness of the CoCrMo femoral heads (R_a = 4.6 nm) was statistically significantly higher than for the zirconia heads (R_a = 3.1 nm). Over the wear test the CoCrMo heads got statistically significantly rougher (R_a = 10.5 nm) whilst the zirconia heads showed no statistically signficant change. The three explanted CoCrMo femoral heads had initial mean surface roughness, R_a, values of 19, 24 and 55 nm with corresponding cup wear rates of 97.6, 131.2 and 148.4 mm³/10⁶ cycles respectively. The very high wear rates against the explanted heads highlight the need for scratch resistant femoral head surfaces.     

Elevated cytokine expression of different PEEK wear particles compared to UHMWPE in vivo

Due to their mechanical properties, there has been growing interest in poly-ether-ether-ketone (PEEK) and its composites as bearing material in total and unicompartmental knee arthroplasty. The aim of this study was to analyze the biological activity of wear particles of two different (pitch and PAN) carbon-fiber-reinforced- (CFR-) PEEK varieties in comparison to ultra-high-molecular-weight-polyethylene (UHMWPE) in vivo. The authors hypothesized no difference between the used biomaterials. Wear particle suspensions of the particulate biomaterials were injected into knee joints of Balb/c mice, which were sacrificed after seven days. The cytokine expression (IL-1β, IL-6, TNF-α) was analyzed immunohistochemically in the synovial layer, the adjacent bone marrow and the articular cartilage. Especially in the bone marrow of the two CFR-PEEK varieties there were increased cytokine expressions compared to the control and UHMWPE group. Furthermore, in the articular cartilage the CFR-PEEK pitch group showed an enhanced cytokine expression, which could be a negative predictor for the use in unicondylar knee systems. As these data suggest an increased proinflammatory potential of CFR-PEEK and its composites in vivo, the initial hypothesis had to be refuted. Summarizing these results, CFR-PEEK seems not to be an attractive alternative to UHMWPE as a bearing material, especially in unicompartmental knee arthroplasty.     

The wear and arc erosion behavior of novel copper based functionally graded electrical contact materials fabricated by hot pressing assisted electroless plating

In this study, hot pressing was used to fabricate the novel copper based functionally graded electrical contact materials synthesized by silver, nickel and chrome plated copper core particles. For the fabrication of functionally graded materials (FGMs) by hot-pressing; pure copper, two and three layered metallic powders were used in the lower, middle and upper layer, respectively. The wear and arc-erosion performances of the developed materials increased from 3 to 10 times as compared to that of pure copper. Wear tests showed that the abrasive wear mechanism was dominant for the FGMs including electroless nickel and chromium coating layer. The highest specific wear rate (SWR) value was found in Cu_f-Ag′ (4.9 × 10⁻⁴ mm³/Nm) materials under the load of 20 N while the lowest SWR value belongs to Cu_f-Ag-Ni′ (2.3 × 10⁻⁴ mm³/Nm) materials under the load of 10 N. While severely melted and deformed regions are dominant on arc erosion surfaces of pure copper and copper-silver containing contacts, flatter and relatively less melted regions were detected on the surfaces of FGMs containing nickel and chromium. The arc-erosion loss values (cm³ × 10⁻⁴) measured for FGM₁ sample were 0.75 and 0.70, 0.99 and 0.88, 1.21 and 1.04 at 3000 cycles under the current of 5 A, 10 A and 15 A in fixed and moving contacts, respectively.     

Quantitative analysis of the wear and wear debris from low and high carbon content cobalt chrome alloys used in metal on metal total hip replacements

The biological reactions to polyethylene wear debris have been shown to result in osteolysis and loosening of total hip arthroplasties. This has led to renewed interest in the use of metal on metal bearings in hip prostheses. This study employed uniaxial and biaxial multistation pin on plate reciprocators to assess how the carbon content of the cobalt chrome alloy and the types of motion affected the wear performance of the bearing surfaces and the morphology of the wear debris generated.The low carbon specimens demonstrated higher wear factors than both the mixed carbon pairings and the high carbon pairings. The biaxial motion decreased the wear rates of all specimens. Plate wear was significantly reduced by the biaxial motion, compared to pin wear. The metal wear particles isolated were an order of magnitude smaller than polyethylene particles, at 60–90 nm, and consequently, 100-fold more particles were produced per unit volume of wear compared to polyethylene. The low carbon specimens produced significantly larger particles than the other material combinations, although it is thought unlikely that the difference would be biologically significant in vivo.The volumetric wear rates were affected by the carbon content of the cobalt chrome alloy, the material combination used and type of motion applied. However, particle morphology was not affected by the carbon content of the alloy or the type of motion applied. ©©1999©Kluwer Academic Publishers     

A novel technique for the detailed size characterization of wear debris

The accurate and detailed characterization of artificial joint wear debris is important in determining both the wear rate of prostheses and understanding the role that the debris plays in the development and progression of aseptic loosening. The novel application of low angle laser light scattering (LALLS) to the particle size characterization of ultra high molecular weight polyethylene (UHMWPE) wear debris is described. The results demonstrate that both ex vivo and in vitro origin wear debris samples, at concentrations typical of those produced via an alkali-digestion retrieval route, can be reproducibly analyzed via LALLS. Because the LALLS route enables particle size analysis of the entire debris sample to be acquired non-destructively and whilst in suspension, artefacts associated with filtering, drying and agglomeration of debris are avoided, in contrast to currently used techniques such as filtration and scanning electron microscopy (SEM) observation. ©2000 Kluwer Academic Publishers     

Sliding wear properties of high purity copper in cryogenic environment

In an effort to understand the influence of cryogenic environment on the friction and wear properties of metallic materials, we performed a series of experiments on high purity work hardened copper (Cu) samples in liquid nitrogen (LN₂) environment against steel (bearing grade; SAE 52100) at varying loads and sliding speeds. The load was varied between 10 and 20 N and sliding speed was varied between 0.89 and 1.34 m/s. In our experiments, a reduction in the steady state coefficient of friction (μ_F) was noted with increasing load (10, 15, 20 N) at the highest sliding speed of 1.34 m/s. High wear rate of the order of 10⁻⁴ mm³/Nm was recorded, which was found to be independent of the load/sliding speed. On the basis of the experimental data and the characteristics of the worn surfaces it is confirmed that significant damage accumulation and plowing-induced material removal contribute to the wear losses. It is noteworthy that oxidative wear or mechanically mixed layer (no transfer from steel counter-body) did not occur to any significant extent under the chosen sliding conditions. The characteristics of the wear damage as a result of cryogenic sliding have been discussed with reference to the prevailing stress conditions and contact temperature.     

The prediction of polyethylene wear rate and debris morphology produced by microscopic asperities on femoral heads

Counterface damage in the form of scratches, caused by bone cement, bone or metallic particles, has been cited as a cause of increased wear of ultra-high molecular weight polyethylene (UHMWPE) acetabular cups. It is known that high levels of particulate wear debris lead to osteolysis. Surface damage was characterized in a series of explanted Charnley femoral heads. The heads had a mean scratch height of 1 m with a mean aspect ratio (defined as height divided by half width) of 0.1. Wear discs were artificially scratched using these scratch geometries as a guide. In addition, the scratch geometries were incorporated into a finite element model of a stainless steel asperity repeatedly sliding over UHMWPE under conditions similar to those in an artificial hip joint. Wear tests showed a strong correlation between the average cross-sectional area of the scratch lip above the mean zero line and the measured wear factor. The finite element model predicted increases in the area of UHMWPE suffering plastic strain with increases in the cross-sectional area of the asperity above the mean line. Analysis of the wear debris showed the mode of the particle size was 0.01–0.5 m for all cases. The morphology of the particles varied with aspect ratio of the asperity, with an increased percentage mass of submicrometer-sized debris with increased scratch lip aspect ratio. The finite element results predicted that the maximum surface strains would increase with increasing asperity aspect ratio. Examination of the worn UHMWPE pin surfaces showed an association between increased surface damage, probably due to high surface strains, and increased aspect ratio. The large areas of surface plastic strain predicted for asperities with high cross-sectional areas above the mean line offer an explanation for the positive correlation between wear rate and the average cross-sectional area of the scratch lip material. The higher surface strains predicted for the higher aspect ratios may explain the increased percentage mass of biologically active submicrometer-sized wear particles found for scratch lips with higher aspect ratios. ©©2000 Kluwer Academic Publishers     

Role of zirconia filler on friction and dry sliding wear behaviour of bismaleimide nanocomposites

This paper discusses the friction and dry sliding wear behaviour of nano-zirconia (nano-ZrO₂) filled bismleimide (BMI) composites. Nano-ZrO₂ filled BMI composites, containing 0.5, 1, 5 and 10 wt.% were prepared using high shear mixer. The influence of these particles on the microhardness, friction and dry sliding wear behaviour were measured with microhardness tester and pin-on-disc wear apparatus. The experimental results indicated that the frictional coefficient and specific wear rate of BMI can be reduced at rather low concentration of nano-ZrO₂. The lowest specific wear rate of 4 × 10⁻⁶ mm³/Nm was observed for 5 wt.% nano-ZrO₂ filled composite which is decreased by 78% as compared to the neat BMI. The incorporation of nano-ZrO₂ particles leads to an increased hardness of BMI and wear performance of the composites shows good correlation with the hardness up to 5 wt.% of filler loading. The results have been supplemented with scanning electron micrographs to help understand the possible wear mechanisms.