Wear Simulator Studies Research Articles

In Vitro Wear Behavior of Knee Implants at Different Load Levels: The Impact of the Test Fluid

Calf serum is defined as a test fluid for in vitro knee wear simulation studies in the ISO standard. However, protein degradation typically occurs during in vitro wear simulation. The current study should indicate whether increased loads change the rheological properties of the test fluid and may, therefore, lead to favorable tribological behavior and reduced wear. Three different load levels were simulated in a displacement-controlled knee wear simulation study. The gravimetric wear rates were determined, pressure measurements were performed, and the dynamic viscosity of the test fluids were analyzed after the simulation of 0.5 × 106 cycles. The lowest load level led to the lowest wear rate, and the lowest contact pressure and contact area, compared to the medium and high-load level. Although, the high-load level led to the highest contact pressure and contact area, the wear rates were comparable to the medium-load level. The rheological measurements revealed the highest dynamic viscosity for the high-load level and no differences could be found between the medium and low loading condition. To perform realistic wear simulation studies, the reproduction of the in vivo interrelationships between the shear forces and wear are necessary.

A Comparison of Wear Patterns on Retrieved and Simulator-Tested Total Knee Replacements.

Aseptic implant loosening is the most common reason for revision surgery after total knee replacement. This is associated with adverse biological reactions to wear debris from the articulating implant components. To predict the amount of wear debris generated in situ, standard wear testing of total knee replacement (TKR) is carried out before its clinical use. However, wear data reported on retrievals of total knee replacement (TKR) revealed significant discrepancies compared with standard wear simulator studies. Therefore, the aim of the present study was to compare the wear patterns on identical posterior-cruciate-retaining TKR designs by analyzing retrieved and experimentally tested implants. The identification and classification of wear patterns were performed using 21 retrieved ultra-high-molecular-weight-polyethylene (UHMW-PE) inserts and four sets of inserts of identical design and material tested in a knee wear simulator. These four sets had undergone different worst-case conditions and a standard test in a wear simulator according to ISO 14243-1. Macroscopic and microscopic examinations of the polyethylene inserts were performed, including the determination of seven modes of wear that correspond to specific wear patterns, the calculation of wear areas, and the classification of the damage over the whole articulating area. Retrieved and standard wear simulator-tested UHMW-PE inserts showed significant differences in wear area and patterns. The total wear areas and the damage score were significantly larger on the retrievals (52.3% versus 23.9%, 32.7 versus 22.7). Furthermore, the range of wear patterns found on the retrievals was not reproducible in the simulator-tested inserts. However, good correspondence was found with the simulator-tested polyethylene inserts under worst-case conditions (third body wear), i.e., deep wear areas could be replicated according to the in vivo situation compared with other wear test scenarios. Based on the findings presented here, standard simulator testing can be used to directly compare different TKR designs but is limited in the prediction of their in situ wear. Preclinical wear testing may be adjusted by worst-case conditions to improve the prediction of in situ performance of total knee implants in the future.

Influence of Metallic Deposition on Ceramic Femoral Heads on the Wear Behavior of Artificial Hip Joints: A Simulator Study.

Several retrieval studies have reported on metallic depositions on ceramic femoral heads, but the effect on the wear behavior of artificial hip joints has not been investigated in wear simulator studies. In the present study, retrieved ceramic heads with metallic depositions as third particles were tested against cross-linked ultra-high-molecular-weight polyethylene (UHMWPE) liners in a hip wear simulator. The amount of liner wear and expansion of metallic depositions on the heads were determined before and after wear testing with digital microscopy. The surface roughness of the heads was investigated in areas with and without metallic depositions by laser scanning microscopy. After five million load cycles, a non-significant reduction in the metallic formation on the retrieved heads was found. The metallic areas showed a higher surface roughness compared to unconcerned areas. The liners showed a higher wear rate of 1.57 ± 1.36 mg/million cycles for 28 mm heads and 2.42 ± 0.82 mg/million cycles for 36 mm heads with metallic depositions, in comparison with new ceramic heads with a 28 mm size ((−0.06 ± 0.89) mg/million cycles) and 36 mm size ((2.04 ± 0.46) mg/million cycles). Metallic transfer on ceramic heads can lead to an increased surface roughness and higher wear rates at the UHMWPE liners. Therefore, metallic contact of the ceramic femoral head should be avoided.

A Systematic Review of Unsystematic Total Ankle Replacement Wear Evaluations.

Numerous studies have reported the use of laboratory multistation joint simulators to successfully predict wear performance and functionality of hip and knee replacements. In contrast, few studies in the peer-reviewed literature have used joint simulation to quantify the wear performance and functionality of ankle replacements. We performed a systematic review of the literature on joint simulator studies that quantified polyethylene wear in total ankle arthroplasty. In addition to the quantified wear results, the load and motion parameters were identified and compared among the studies. A search was performed according to the PRISMA (Preferred Reporting Items for Systematic Reviews and Meta-Analyses) guidelines to identify articles reporting total ankle replacement polyethylene wear using joint simulators. Nine studies that used joint simulators and 1 study that used a computer simulation were found. Although all studies used physiological multidirectional motions (i.e., internal/external rotation, plantar flexion/dorsiflexion, anterior/posterior translation), there was large variability among the studies in the magnitudes of these motions. Among these studies, mean non-cross-linked polyethylene wear ranged from 3.3 ± 0.4 to 25.8 ± 3.1 mm per million cycles. In contrast, mean highly cross-linked polyethylene wear ranged from 2.1 ± 0.3 to 3.3 ± 0.4 mm per million cycles. The wide distribution in wear rates was attributable to the highly inconsistent kinematic parameters and loads applied as well as differences in implant design and materials. There is a severe lack of clinically applicable data on wear performance of total ankle replacements in the peer-reviewed literature. No universal set of kinematic load parameters has been established. Furthermore, only 2 of the published studies have validated their findings using independently derived data, such as retrieval analysis. These shortcomings make it difficult to compare findings as a function of design parameters and materials, or to draw clinically relevant conclusions from these simulations. More work is required to enhance the predictive capability of in vitro simulations of total ankle replacements. The results of joint wear simulator studies may not accurately represent in vivo wear of total ankle replacements. Joint simulator studies should establish that they are accurately replicating in vivo wear, thus enabling use of their predictive capabilities for new materials and designs.

The effect of vitamin E–enhanced cross-linked polyethylene on wear in shoulder arthroplasty—a wear simulator study

Wear of the polyethylene glenoid component and subsequent particle-induced osteolysis remains one of the most important modes of failure of total shoulder arthroplasty. Vitamin E is added to polyethylene to act as an antioxidant to stabilize free radicals that exist as a byproduct of irradiation used to induce cross-linking. This study was performed to assess the invitro performance of vitamin E-enhanced polyethylene compared with conventional polyethylene in a shoulder simulator model. Vitamin E-enhanced, highly cross-linked glenoid components were compared with conventional ultrahigh-molecular-weight polyethylene glenoids, both articulating with a ceramic humeral head component using a shoulder joint simulator over 500,000 cycles. Unaged and artificially aged comparisons were performed. Volumetric wear was assessed by gravimetric measurement, and wear particle analysis was also subsequently performed. Vitamin E-enhanced polyethylene glenoid components were found to have significantly reduced wear rates compared with conventional polyethylene in both unaged (36% reduction) and artificially aged (49% reduction) comparisons. There were no differences detected in wear particle analysis between the 2 groups. Vitamin E-enhanced polyethylene demonstrates improved wear compared with conventional polyethylene in both unaged and artificially aged comparisons and may have clinically relevant benefits.

Investigation of characteristic rippling topology produced during UHMWPE sliding

Ultra-high molecular weight polyethylene (UHMWPE) remains the sole polymer bearing material for use in orthopedic implants and its behavior has been studied extensively. Various techniques, such as crosslinking, have been employed to reduce the total wear volume. However, little is known about the actual wear mechanisms that contribute to the removal of debris from the bulk. A nearly universal observation of the wear surfaces after multidirectional sliding of UHMWPE is the presence of regularly-spaced parallel ripples whose wavelength closely matches debris size. In spite of the fact that this phenomenon has been observed for decades in both retrieved implants as well as wear simulation studies, the specific causes of the rippled topology have not yet been identified. Proposed mechanisms have included abrasion, crystallinity effects, fatigue, stick-slip behavior, thermal softening, and Schallamach waves. This investigation involved an experimental and computational factorial test design to determine the role of various parameters involved in the rippling phenomenon. The authors employed a film buckling theoretical model and applied it to UHMWPE using a finite element modeling approach. Wear testing was performed on a two-axis tribometer to expose the polymer to multi-directional sliding. Characteristic ripples developed on all wear surfaces, but with varying periodicity. Two regimes of the ripple phenomenon were observed: localized long wavelength (2.2–3.2 µm) ripples which appeared to dominate before steady state wear was established, and long-range shorter-wavelength (approximately 1 µm) ripples which indicated a steady-state condition. Surprisingly, feature wavelength showed little dependence on roughness or temperature but were slightly affected by contact pressure. The wear surfaces were cleaved and examined by SEM to gauge the thickness of the plastically deformed quasi-film layer. These layers were on the order of 10–20 nm thick. Further examination of the surfaces confirms the development of a deformed layer which acts similarly to a buckled thin film.

Why Coating Technologies for Hip Replacement Systems, and the Importance of Testing Them In Vitro

The rationale behind novel surface engineering and coating technologies for implant articular surfaces is described especially for the significant number of metal-sensitive patients with total hip replacement. We list the important coating characteristics, and emphasize adhesion as the most challenging. We report on 2 large scale experimental hip wear simulator studies we conducted on samples of 2 such coating technologies implemented by external experts in their respective fields. One was a titanium nitride ceramic coating on a metal-on-metal total hip replacement, which aimed at wear reduction and a barrier against metal-ion diffusion. The other was a nanocrystalline homometallic surface treatment where same element metals were vapor-deposited in a novel ion-beam-assisted deposition process onto CoCr alloy femoral heads in CoCr metal on ultra high molecular weight polyethylene (UHMWPE) hips. The gradual reduction in crystal size toward the surface into nano size aimed at increasing hardness and reducing wear. Both test series were conducted on an AMTI hip simulator for 5 million cycles under the 14242-1 ISO standard, simulating walking gait, without any deliberate edge-loading or harsh conditions, and using uncoated specimens of identical design and materials as controls in each test. Both tests resulted in some of the coating eventually rubbing away (separating) to various levels on all specimens. The wear rate of the coated specimens was generally higher not lower than the uncoated controls, but not multiply so. Our discussion compares and contrasts our results with others, and we speculate whether a metal-sensitive arthritic patient who badly needs a hip replacement may still benefit from a coated implant despite the risk.

Contact modeling in boundary element analysis including the simulation of thermomechanical wear

Computational mechanics codes include contact or over-closure models to account for contact stiffness, which can be related to surface roughness and nominal pressure from Greenwood and Williamson. We present results of wear simulation studies with adjustable contact parameters for a ring-on-ring sliding configuration. A boundary element code is used to solve the non-linear contact problem with an axisymmetric thermoelastic representation of the rings, along with a localized Archard wear model. Parametric studies indicate the extent to which the wear tracks can be affected by the roughness and wear history. The effects range from minor to moderately significant, with areas of maximum wear shifting radially. Contact parameters influence pressure, temperature and wear distributions, with stiffer contacts resulting in greater localization.

The Divergence of Wear Propagation and Stress at Steep Acetabular Cup Positions Using Ceramic Heads and Sequentially Cross-Linked Polyethylene Liners

The aim of the present wear simulator study was to assess the effect of steep acetabular cup positions on the wear propagation of highly cross-linked-PE (HX-PE) liners. Furthermore, a finite element analysis (FEA) was performed in order to calculate the stress within the HX-PE material in case of steep cup positions under physiological loadings. The higher stress in the HX-PE at a steep acetabular cup position did not result in increased wear in the present wear simulator study. The gravimetrical wear rates at normal (45°) and steep cup inclinations (75°) showed wear amounts of 3.15±0.27mg and 2.18±0.31mg per million cycles (p=0.028), respectively. However, FEA revealed clear increase in stress at the HX-PE liners with respect to steep cup positions.

Synovial fluid replication in knee wear testing: an investigation of the fluid volume.

Wear testing cannot replicate the variations in wear rates and wear mechanisms seen in vivo, which may be related to differences between in vivo and in vitro conditions. A considerable difference exists between the in vivo synovial fluid volume (few milliliter) and the in vitro substituted bovine serum volume (several hundred milliliter). The aim of this study was to analyze the effects of a reduced fluid volume on the wear behavior in a knee wear simulator study. Four wear tests with decreasing fluid volumes (250, 150, 75, and 45 ml) were carried out. Using a large fluid volume of 250 ml for wear testing resulted in a wear rate of 9.7±1.2 mm3/10(6) cycles. Decreasing the fluid volume consecutively reduced the wear rate to down to 8.8±1.4 mm3/10(6) for 150 ml (p=1.00), 5.6±1.2 mm3/10(6) for 75 ml (p=0.01), and 1.0±0.2 mm3/10(6) cycles for 45 ml fluid volume (p≤0.01). Additionally, higher serum degradation and larger wear particles were observed with smaller fluid volumes used for testing. This study demonstrates the high relevance of the protein-based lubricant on the wear behavior and the technical limitation to replicate the synovial fluid in simulator tests. Wear testing should be carried out using larger fluid volumes (e.g., 250 ml) to generate physiological relevant wear masses.

Long-Term Performance of Ceramic and Metal Femoral Heads on Conventional Polyethylene in Young and Active Patients

Ceramic femoral heads produce less wear of the opposing polyethylene than do metal femoral heads in wear simulation studies. This is a matched-pair analysis of the wear of ceramic and metal femoral heads on conventional polyethylene in uncemented total hip replacements in young, active patients at a minimum of fifteen years of follow-up. From June 1989 to May 1992, thirty-one matched pairs of alumina ceramic or cobalt-chromium metal femoral heads were identified. Patients were matched on the basis of age, sex, body weight, diagnosis, and activity level. The mean age was 55 ± 9 years (range, twenty-three to sixty-five years) at the time of surgery. All procedures were performed with a posterolateral surgical approach by a single surgeon using press-fit Ranawat-Bernstein femoral stems, Harris-Galante-II acetabular cups, GUR 4150 conventional polyethylene (sterilized in argon), and 28-mm-diameter femoral heads. Wear measurements were performed by two independent observers using the computer-assisted Roman software. The average duration of follow-up was 17 ± 1.7 years (range, fifteen to twenty years). The mean Hospital for Special Surgery hip scores (and standard deviation) in the ceramic and metal groups were 39 ± 4 and 40 ± 3 at the time of final follow-up. The University of California Los Angeles activity score at the time of the final follow-up was 6 ± 2 for both groups. The mean wear rates for the ceramic group and the metal group were 0.086 ± 0.05 mm/yr and 0.137 ± 0.05 mm/yr, respectively (p = 0.0015). There was one reoperation in the ceramic group because of distal femoral osteolysis. There were three failures in the metal group, requiring isolated liner exchange in two hips and revision of the acetabular component in one hip because of wear-induced osteolysis and/or loosening that caused symptoms. Five hips in the ceramic group and six hips in the metal group had radiographic evidence of acetabular or femoral osteolysis, but none were symptomatic. Ceramic femoral heads produced significantly less wear on conventional polyethylene liners at the time of long-term follow-up than did metal heads in this matched-pair analysis of young and active patients with uncemented fixation.

The influence of third-body particles on wear rate in unicondylar knee arthroplasty: a wear simulator study with bone and cement debris

The reduced intraoperative visibility of minimally invasive implanted unicondylar knee arthroplasty makes it difficult to remove bone and cement debris, which have been reported on the surface of damaged and retrieved bearings. Therefore, the aim of this study was to analyze the influence of bone and cement particles on the wear rate of unicompartmental knee prostheses in vitro. Fixed bearing unicompartmental knee prostheses were tested using a knee-wear-simulator according to the ISO standard 14243-1:2002(E) for 5.0 million cycles. Afterwards bone debris (particle size 671 ± 262 μm) were added to the test fluid in a concentration of 5 g/l for 1.5 million cycles, followed by 1.5 million cycles blended with cement debris (particle size 644 ± 186 μm) in the same concentration. Wear rate, knee-kinematics and wear-pattern were analyzed. The wear rate reached 12.5 ± 1.0 mm³/million cycles in the running-in and decreased during the steady state phase to 4.4 ± 0.91 mm³/million cycles. Bone particles resulted in a wear rate of 3.0 ± 1.27 mm³/million cycles with no influence on the wear rate compared to the steady state phase. Cement particles, however, lead to a significantly higher wear rate (25.0 ± 16.93 mm³/million cycles) compared to the steady state phase. The careful removal of extruded cement debris during implantation may help in reducing wear rate. Bone debris are suggested to have less critical influence on the prostheses wear rate.

Patient Activity and Hip Wear

Polyethylene wear rates in vivo are highly variable. A major factor contributing to the variability in clinical wear rates is patient activity. Pedometer studies have documented a 45-fold range in patient activity. A microprocessor worn on the ankle is more accurate than a pedometer and records the motion of the leg in real time, allowing the assessment of walking speed and pattern. Average patient activity approaches 2 million gait cycles/year. With adjustments for patient weight and activity, in vivo wear studies are highly correlated to pre-clinical hip wear simulator studies. Walking speed and pattern also influence polyethylene wear. The greatest patient activity and wear occur during the first 5 years post-implantation. Walking number of gait cycles and walking speed both decrease with aging, resulting in decreasing wear over time. The low wear rates of cross-linked polyethylene combined with decreasing patient activity with aging indicate a reduced risk of wear-related complications over the long term.

A hip simulator study of metal-on-metal hip joint device using acetabular cups with different fixation surface conditions

In vitro wear data for hip joint devices reported in the literature vary in a wide range from one simulator study to another sometimes for the same type of device tested under identical physiological testing conditions. We hypothesized that non-bearing surface condition of the testing components could be an important factor affecting the simulator wear results. To confirm this hypothesis, fifteen 50 mm metal-on-metal hip resurfacing devices with identical bearing specifications were tested in a ProSim hip wear simulator for 5 million cycles. The heads were standard Birmingham Hip Resurfacing (BHR) heads; whilst the pairing acetabular cups were identical to the standard BHR cup except their different back surface conditions, including: (a) off-the-shelf products after removing the hydroxyapatite (HA) coating; (b) semi-finished products without HA coating; and (c) purposely-made cups without cast-in beads and HA coating. Results showed that the different back surfaces of the cups used indeed caused significantly large variations in the gravimetrically measured wear loss. We postulated that materials loss from the non-bearing surface of the testing components could contribute to the gravimetrically measured wear loss during a wear simulator test both directly and indirectly. The results presented in this paper pertain to In vitro wear simulator study and have little clinical relevance to the performance of any implant in vivo.

In Vitro Quantification of Wear in Tibial Inserts Using Microcomputed Tomography

Wear of polyethylene tibial inserts can decrease the longevity of total knee arthroplasty. Wear is currently assessed using laboratory methods that may not permit backside wear measurements or do not quantify surface deviation. We developed and validated a technique to quantify polyethylene wear in tibial inserts using microcomputed tomography (micro-CT), a nondestructive high-resolution imaging technique that provides detailed images of surface geometry in addition to volumetric measurements. Six unworn and six wear-simulated polyethylene tibial inserts were evaluated. Each insert was scanned three times using micro-CT at a resolution of 50 μm. The insert surface was reconstructed for each scan and the insert volume was calculated. Gravimetric analysis was performed for all inserts, and the micro-CT and gravimetric volumes were compared to determine accuracy. We created three-dimensional surface deviation maps. Micro-CT generated high-quality three-dimensional renderings of the insert surface geometry. Between-scan precision was 0.07%; we observed no difference between micro-CT and gravimetric volume measurements. Micro-CT can provide precise and accurate volumetric measurements in addition to quantifiable three-dimensional surface deviation maps for the entire insert surface. The technique has the potential to evaluate wear in wear simulator trials and retrieval studies. This micro-CT technique combines the benefits of volumetric and surface scanning methods to quantify wear across all surfaces of polyethylene components with a single tool. When applied in wear simulator and retrieval studies, these measurements can be used to evaluate and predict the wear properties of the components.

Characterization of protein degradation in serum‐based lubricants during simulation wear testing of metal‐on‐metal hip prostheses

A size exclusion high performance liquid chromatography (SEC-HPLC) method has been developed which is capable of separation and quantitation of bovine serum albumin (BSA) and bovine serum globulin (BSG) components of serum-based lubricant (SBL) solutions. This allowed characterization of the stability profiles of these proteins when acting as lubricants during hip wear simulation, and identification of wear-specific mechanisms of degradation. Using cobalt-chromium metal-on-metal (MOM) hip joints, it was observed that BSA remained stable for up to 3 days (215K cycles) of wear testing after which the protein degraded in a fairly linear fashion. BSG on the other hand, began to degrade immediately and in a linear fashion with a rate constant of 5% per day. Loss of both proteins occurred via the formation of high molecular weight aggregates which precipitated out of solution. No fragmentation of the polypeptide backbone of either protein was observed. Data obtained suggest that protein degradation was not due to microbial contamination, denaturation at the air-water interface, or frictional heating of articulating joint surfaces in these studies. We conclude that the primary source of protein degradation during MOM simulation testing occurs via high shear rates experienced by SBL solutions at articulating surfaces, possibly coupled with metal-protein interactions occurring as new and reactive metal surfaces are generated during wear testing. The development of this analytical methodology will allow new studies to clarify the role of SBL solutions in wear simulation studies and the interactions and lubricating properties of serum proteins with prosthetic surfaces other than MOM.

Biotribology of alternative bearing materials for unicompartmental knee arthroplasty

The objective of our wear simulator study was to evaluate the suitability of two different carbon fibre-reinforced poly-ether-ether-ketone (CFR-PEEK) materials for fixed bearing unicompartmental knee articulations with low congruency. In vitro wear simulation was performed according to ISO 14243-1:2002 (E) with the clinically introduced Univation F fixed bearing unicompartmental knee design (Aesculap AG, Tuttlingen, Germany) made of UHMWPE/CoCr29Mo6 in a direct comparison to experimental gliding surfaces made of CFR-PEEK pitch and CFR-PEEK PAN. Gliding surfaces of each bearing material (n=6+2) were gamma-irradiated, artificially aged and tested for 5 million cycles with a customized four-station knee wear simulator (EndoLab, Thansau, Germany). Volumetric wear assessment, optical surface characterization and an estimation of particle size and morphology were performed. The volumetric wear rate of the reference PE1-6 was 8.6 +/- 2.17 mm(3) per million cycles, compared to 5.1+/-2.29 mm(3) per million cycles for PITCH1-6 and 5.2 +/- 6.92 mm(3) per million cycles for PAN1-6; these differences were not statistically significant. From our observations, we conclude that CFR-PEEK PAN is obviously unsuitable as a bearing material for fixed bearing knee articulations with low congruency, and CFR-PEEK pitch also cannot be recommended as it remains doubtful wether it reduces wear compared to polyethylene. In the fixed bearing unicompartmental knee arthroplasty examined, application threshold conditions for the biotribiological behaviour of CFR-PEEK bearing materials have been established. Further in vitro wear simulations are necessary to establish knee design criteria in order to take advantage of the biotribiological properties of CFR-PEEK pitch for its beneficial use to patients.

Regional Measurements of Surface Deviation Volume in Worn Polyethylene Joint Replacement Components

Total joint replacements can be subject to the loss of polyethylene material due to wear, leading to osteolysis and decreased implant longevity. Micro-computed tomography (micro-CT) techniques have recently been developed to calculate 3D surface deviations in worn implant components. We describe a micro-CT technique to measure the volume of the surface deviations (volume of wear plus creep) within a specific region or compartment, and report its repeatability and reproducibility. Six worn polyethylene tibial inserts were scanned using a laboratory micro-CT scanner and subsequently reconstructed at 50 μm voxel spacing. A previously developed custom software application was used to quantify the 3D surface deviations between the worn tibial inserts and an unworn reference geometry. Three observers (two trained and one expert) used new custom software to manually outline the localized regions of surface deviation (three times for each of the worn inserts) and calculate the volume of the deviations. The overall intraobserver variability in the surface deviation volumes was 3.6% medially and 1.1% laterally. The overall interobserver variability was 4.8% medially and 1.7% laterally. Placement of points in outlining the region of deviation contributed the greatest variability to the measurements. Repeatability and reproducibility of the volume measurements are similar to measurements of total (nonregional) wear volume including a previous micro-CT technique (10%), fluid displacement (4.8%), and radiographic measurements (15.7%). The principles of this technique can likely be used to measure regional wear and creep volume in knee and hip joint replacement components from wear simulator, pin-on-disk, and retrieval studies.

Three-Dimensional Surface Deviation Maps for Analysis of Retrieved Polyethylene Acetabular Liners Using Micro-Computed Tomography

Micro-computed tomography (micro-CT) has previously been validated for measuring wear volume in polyethylene acetabular liners. The creation of 3-dimensional (3D) articular and backside surface deviation maps using micro-CT is described. An acetabular liner was retrieved after 16.7 years of implantation and scanned with micro-CT along with a new, never-implanted liner of the same size and type. The liner surface geometries were reconstructed and co-aligned. A 3D comparison and cross-sectional analysis was performed. Maximum 3D deviation of the articular surface was −2.48 ± 0.02 mm, with maximum backside deviation of 0.46 ± 0.02 mm. Micro-CT can measure surface deviation and therefore calculate the volume of wear plus creep of retrieved acetabular liners, and may be applicable for wear simulator studies and analyzing other polyethylene components including tibial inserts.

Consideration of non-bearing surface condition and its potential effect on hip wear simulation test results

Material loss during hip simulator studies is commonly attributed to wear occurring within the bearing surfaces. Potential mass loss from the non-bearing surfaces and its contribution to the overall gravimetric measurement results are seldom mentioned in the literature. In this study, CoCrMo alloy disc samples and resurfacing cups with various surface conditions were immersion tested in serum solution for up to 1200h. Gravimetric measurement results showed that the static immersion and cleaning induced weight loss for a cup alone could be in the same order of magnitude as a typical hip simulator tested wear loss reported in the literature for a metal-on-metal hip joint. Since in a hip wear simulator the parts to be tested are generally immersed in serum or other physiologically relevant solutions and cleaned periodically before gravimetric measurement, it is highly possible that material loss can also occur from the non-bearing surfaces, affecting the accuracy, repeatability and comparability of hip simulation tested wear results. Accordingly, non-bearing surface conditions and the potential material loss from the non-bearing surfaces have to be considered in designing hip simulator test protocols and in analysing wear results. The results presented in this paper pertain to in vitro wear simulator studies and have little clinical relevance to the performance of any implant in vivo.