Mechanistic and morphological origins of ultra-high molecular weight polyethylene wear debris in total joint replacement prostheses

Sensitivity, robustness, and reproducibility of U-shaped delamination test for evaluation of candidate ultra-high molecular weight polyethylene materials for joint replacements

The delamination of ultra-high molecular weight polyethylene (UHMWPE) in artificial joints is a major cause limiting the long-term clinical results of arthroplasty. However, the conventional test method using simple reciprocation to evaluate the delamination resistance of UHMWPE materials has insufficient detection sensitivity. To reproduce delamination, the unconformity contact must be maintained throughout the test so that the maximum stress is generated below the surface. Therefore, a test method that applies a U-shaped motion comprising two long-linear and one short linear sliding motion was developed. The sensitivity, robustness, and reproducibility of the U-shaped delamination test were investigated and compared with the traditional test method. The traditional test method could reproduce delamination only in materials that had degraded considerably, whereas the U-shaped delamination test could reproduce delamination in a wide range of materials, demonstrating its superior sensitivity. Additionally, using a higher load helped accelerate the test without affecting the test results. The optimal length of the short linear sliding motion was confirmed to be 1 mm. Finally, the inter-laboratory reproducibility of the U-shaped delamination test was confirmed using the round-robin test. The U-shaped delamination test demonstrates high sensitivity, robustness, and reproducibility and contributes to the selection and development of UHMWPE materials and artificial joints with a lower risk of delamination.

Statistical analysis of VEXLPE wear against alumina produced by a new 200-station, multidirectional pin-on-disk device

With low wear rates shown by contemporary bearing materials of total hip prostheses, the standard deviation of wear rate is relatively high. Therefore, large sample sizes are needed for an adequate power of test. Because wear tests take a long time, it is practical to test several samples simultaneously. A new high-capacity, multidirectional wear test device, called the SuperCTPOD-200, was introduced. A 3 million-cycle wear test with an unprecedented sample size of 200 was performed for VEXLPE. The duration of the test was 6 weeks. The wear factor was normally distributed with a mean ± SD of 1.64 × 10-7 mm3/Nm ± 0.22 × 10-7 mm3/Nm (n = 200). The observation that SD was 13.1% of the mean can be useful in power analyses of future tests with other highly cross-linked polyethylenes. Burnishing was the most typical feature on the worn pins, which was in agreement with clinical findings on retrieved acetabular liners. The present study emphasizes statistics that often plays a minor role only in wear studies.

Effects of Electron Beam Irradiation on Mechanical and Tribological Properties of PEEK

In this work, the mechanical and tribological characteristics of polyetheretherketone (PEEK) sheets were enhanced by electron beam irradiation. PEEK sheets irradiated at a speed of 0.8 m/min with a total dose of 200 kGy achieved the lowest specific wear rate of 4.57 ± 0,69 (10−6 mm3/N−1m−1), compared to unirradiated PEEK with a rate of 13.1 ± 0.42 (10−6 mm3/N−1m−1). Exposure to an electron beam at 9 m/min for 30 runs, with a dose of 10 kGy per run for a total dose of 300 kGy, resulted in the highest improvement in microhardness, reaching 0.222 GPa. This may be due to the decrease in crystallite size, as indicated by the broadening of the diffraction peaks in the irradiated samples. According to the results of thermogravimetric analysis, the degradation temperature of the irradiated samples remained unchanged at 553 ± 0.5 °C, except a sample irradiated at dose 400 kGy, where the degradation temperature shifted towards a lower position of 544 ± 0.5 °C. Differential scanning calorimetry results revealed that the melting temperature () of the unirradiated PEEK was about 338 ± 0.5 °C, while a high temperature shift of the was observed for the irradiated samples.

A novel hip joint prosthesis with uni-directional articulations for reduced wear

A novel polymer-on-metal hip joint prosthesis design that makes use of uni-directional articulations was developed and tested in this work. The new implant was tested using two polymer variants, virgin ultra-high molecular weight polyethylene (UHMWPE), and Vitamin E-infused highly crosslinked polyethylene (VEHXPE). The degrees of freedom of the ball-and-socket are reproduced by three cylindrical orthogonally-aligned articulations. This unconventional design leverages on the molecular orientation hardening mechanisms of the polyethylene and increased contact area to minimize wear. An experimental hip joint simulator was used to compare the gravimetric wear of the conventional ball-on-socket and the new implant. The new prosthesis including UHMWPE components produced a 78% reduction in wear, whereas the new prosthesis with VEHXPE components produced a 100% reduction in wear, as no measurable wear was detected. Machining marks on the acetabular cups of the new prosthesis were retained for both polyethylene variants, further demonstrating the low levels of wear exhibited by the new implants. Both polyethylene materials produced particles in the range of 0.1-1.0 μm, which are the most biologically active. Nonetheless, the extremely low wear rates are likely to induce minimal osteolysis effects. Furthermore, the novel design also offers an increase of more than 24% in the range of motion in flexion/extension when compared to a dual-mobility hip implant. A prototype of the prosthesis was implanted into a Thiel-embalmed human cadaver during a mock-surgery, which demonstrated high resistance to dislocation and the possibility of performing a figure of four position.

Analysis of hip joint cross-shear under variable activities using a novel virtual joint model within Visual3D

Cross-shear forces occur between bearing surfaces at the hip and have been identified as a key contributor to prosthesis wear. Understanding the variation in relative motion paths between both individuals and activities, is a possible explanation for increased revision rates for younger patients and could assist in improved pre-clinical testing regimes. Additionally, there is little information for the pre-clinical testing of cartilage substitution therapies for younger more active individuals. The calculation of motion paths has previously relied on computational modelling software which can be complex and time-consuming. The aim of this study was to determine whether the motion paths calculations could be integrated into gait analysis software to improve batch processing, reduce analysis time and ultimately improve the efficiency of the analysis of cross-shear variation for a broader range of activities. A novel Virtual Joint model was developed within Visual3D for calculating motion paths. This model was compared to previous computational methods and found to provide a competitive solution for cross shear analysis (accuracy <0.01 mm error between methods). The virtual hip model was subsequently applied to 13 common activities to investigate local aspect ratio's, velocities and accelerations. Surprisingly walking produced the harshest cross shear motion paths in subjects. Within walking, of additional interest was that the localised change in acceleration for subjects was six times greater compared to the same point on an equivalent smoothed simulator cycle. The Virtual hip developed in Visual 3D provides a time saving technique for visualising and processing large data sets directly from motion files. The authors postulate that rather than focussing on a generalised smoothed cross-shear model that pre-clinical testing of more delicate structures should consider localised changes in acceleration as these may be more important in the assessment of cartilage substitutes sensitive to shear.

Friction RandomPOD-A new method for friction measurement in noncyclic, multidirectional, dynamic pin-on-disk tests for orthopaedic bearing materials

The 16-station, computer-controlled RandomPOD wear test device was re-designed into a friction measurement device, Friction RandomPOD. The motion was implemented by a servo-electric x-y-stage and the load was proportional-pneumatic. The direction of sliding, velocity (v), acceleration, and the magnitude of the load (L) varied randomly and continuously. The nominal contact pressure p varied between 0 and 2.4 MPa and v between 0 and 40 mm/s. In the first version of the device, the frictional force components were continuously measured by two miniature load cells in two perpendicular directions. In the second version, the measurement was done with a three-axial, commercial load cell. The resultant frictional force was divided by the instantaneous load in order to obtain the coefficient of friction (μ) at a frequency of 200 Hz. Due to the light and accurate design of the device, vibrations were absent in the measured signals although the measurements were most dynamic. Hence no filtering was needed. Serum lubricated polyethylene/CoCr tests revealed non-symmetric distributions of μ, friction power P_μ, and μ vs. pv.

Comparison of In-Vivo Performance Characteristics of First-Generation and Second-Generation Cross-Linked and Conventional Explants

BACKGROUND

Implants made of ultra high molecular weight polyethylene (UHMWPE) has been used for almost 60 years in hip joint arthroplasty as articulating surface. UHMWPE implants have evolved over time from conventional to cross-linked implants. Chemical, morphologic, and micromechanical characteristics play important roles in overall in vivo performance.

METHODS

This study aimed at comparing chemical, morphologic, and micromechanical in vivo performance characteristics of conventional and cross-linked explants. Optical damage scoring, Fourier transform infrared spectroscopy, differential scanning calorimetry, and depth sensing indentation techniques were used for the measurements. The measurement results were used to correlate oxidation index, degree of crystallinity, E-Modulus, and hardness.

RESULTS

Different manufacturing processes directly affect implants' in vivo performance. Conventional explants are more susceptible to oxidation in the absence of thermal treatment and air ambient gamma irradiation sterilization, whereas they have higher degree of crystallinity and E-Modulus values. Introduction of a thermal treatment step to first-generation cross-linked explants has decreased the oxidation susceptibility but degraded the crystalline structure however, such explants manufactured with moderate total cross-linking irradiation dose and a combination of both remelting and annealing thermal treatment methods are the exceptions. The second-generation cross-linked explants in general have much better E-Modulus, that is, hardness values over all other types of explants and are least susceptible to oxidation.

CONCLUSION

The results suggest that in vivo performances of explants could benefit from the hybrid UHMWPE implant manufacturing techniques, such as moderate cross-linking irradiation doses, remelting, followed by annealing and ethylene oxide or gas plasma sterilization.

In vivo volumetric and linear wear measurement of reverse shoulder arthroplasty at minimum 5-year follow-up

BACKGROUND

Reverse shoulder arthroplasty is quickly becoming the most frequently performed glenohumeral joint replacement. The purpose of this study was to evaluate the volumetric and linear wear rates of ultrahigh-molecular-weight polyethylene humeral liners in vivo at a minimum 5-year follow-up.

METHODS

Radiostereometric analysis was used to image 15 patients at terminal range of motion in forward flexion, abduction, external rotation, and internal rotation and with the arm at the side. The relative position and orientation of the glenosphere and polyethylene were identified for each arm position. The apparent intersection of the glenosphere into the polyethylene was recorded as wear. Mean volumetric and linear wear rates were recorded, and Pearson correlation coefficients were applied to the 36-mm liners to assess the relationship between the wear rate and term of service.

RESULTS

The mean reverse shoulder arthroplasty term of service at the time of imaging was 8 ± 1 years (range, 6-11 years). The mean volumetric and linear wear rates for the 36-mm liners (n = 13) were 42 ± 22 mm³/yr (r = 0.688, P = .009) and 0.11 ± 0.03 mm/yr (r = 0.767, P = .002), respectively. The mean volumetric and linear wear rates for the 42-mm liners (n = 2) were 114 ± 44 mm³/yr and 0.17 ± 0.01 mm/yr, respectively. No single arm position was able to capture all recorded wear individually.

CONCLUSION

This study showed volumetric and linear wear rates of approximately 40 mm³/yr and 0.1 mm/yr, respectively, for the 36-mm polyethylene liners. The 42-mm liners showed higher wear rates, although a greater number of subjects is required for conclusive results. In vivo wear of reverse total shoulder arthroplasty is multidirectional and perceptible.

Design of Wear-Resistant UHMWPE-Based Composites Loaded with Wollastonite Microfibers Treated with Various Silane Coupling Agents

The tribomechanical properties of the wear-resistant ultrahigh molecular weight polyethylene (UHMWPE)-based composites loaded with wollastonite microfibres silanized with various coupling agents (“KH-550”, “Penta-1006”, and “OTS”) were investigated. It was demonstrated that the mechanical properties of UHMWPE-based composites filled with various amounts of wollastonite (7–23 wt. %) increased by 1.3 times (yield strength) and by 1.8 times (elastic modulus), while the wollastonite silanization further improved yield strength by 9% in some cases. It was demonstrated that the composite loaded with 23 wt. % wollastonite silanized with the “KH-550” coupling agent possessed the maximum wear resistance under “moderate” conditions of tribological loading. Under “severe” conditions, the composites containing 23 wt. % wollastonite silanized with the less efficient “OTS” and “Penta-1006” agents showed the greatest wear resistance during dry sliding friction. Wear resistance significantly depended on filler weight fraction and the load–speed mode of the tribological tests. Based on the obtained experimental data on the mechanical (including impact toughness) and tribological properties of the UHMWPE-based composites loaded with wollastonite, the optimal compositions (the filler content and the type of the coupling agent) for two load–speed modes were designed using the developed computer algorithm. The composites provided the predefined high tribomechanical properties for operation in the metal-polymer friction units compared to neat polymer.

Ultra-High-Molecular-Weight-Polyethylene (UHMWPE) as a Promising Polymer Material for Biomedical Applications: A Concise Review

Ultra-High Molecular Weight Polyethylene (UHMWPE) is used in biomedical applications due to its high wear-resistance, ductility, and biocompatibility. A great deal of research in recent decades has focused on further improving its mechanical and tribological performances in order to provide durable implants in patients. Several methods, including irradiation, surface modifications, and reinforcements have been employed to improve the tribological and mechanical performance of UHMWPE. The effect of these modifications on tribological and mechanical performance was discussed in this review.

Finite element analysis of polyethylene wear in total hip replacement: A literature review

Evaluation and prediction of wear play a key role in product design and material selection of total hip replacements, because wear debris is one of the main causes of loosening and failure. Multifactorial clinical or laboratory studies are high cost and require unfeasible timeframes for implant development. Simulation using finite element methods is an efficient and inexpensive alternative to predict wear and pre-screen various parameters. This article presents a comprehensive literature review of the state-of-the-art finite element modelling techniques that have been applied to evaluate wear in polyethylene hip replacement components. A number of knowledge gaps are identified including the need to develop appropriate wear coefficients and the analysis of daily living activities.

Twenty-year survivorship of a cemented mobile bearing Total Knee Arthroplasty

BACKGROUND

Increasing numbers of Total Knee Arthroplasty (TKA) operations are carried out worldwide each year. This brings with it an ever-increasing revision burden and it is therefore important to appreciate both the functional outcome and survivorship of established arthroplasties when considering new designs. We aim to evaluate the long-term survivorship of a fully cemented mobile bearing Total Knee Arthroplasty.

METHODS

This study prospectively analyses the 20-year survivorship of a cohort of 487 consecutive patients who underwent cemented TKA under the care of a single surgeon using the Low Contact Stress (LCS) rotating platform (RP) implant. These patients were followed up prospectively with patient reported and functional outcomes recorded at regular intervals postoperatively.

RESULTS

Five hundred and forty-two consecutive primary TKAs were carried out in 487 patients. A total of 139 knees (25.6%) were reviewed at 20 years post-operation. Overall cumulative survivorship, using revision for any reason as primary endpoint, was 98.0%. Mean Knee Society Scores for the patient cohort were 87.3 (Clinical score) and 52.5 (Functional score). Eleven (2.0%) were revised within 20 years - two for aseptic loosening, two for unexplained pain, five secondary patellar resurfacings for anterior knee pain, one for late infection and one liner exchange following spin-out.

CONCLUSION

This series demonstrates excellent survivorship and satisfactory outcome of a cemented mobile bearing TKA at 20 years.

Effect of wear, acetabular cup inclination angle, load and serum degradation on the friction of a large diameter metal-on-metal hip prosthesis

BACKGROUND

The large-scale clinical problem caused by unacceptable tribological behaviour of certain large diameter metal-on-metal prosthetic hips has directed attention to adverse condition testing. High metal-on-metal wear is connected with adverse reaction to metal debris. Friction is important because high friction may be associated with high wear, risk the fixation of the cup, and cause detrimental heating of periprosthetic tissues.

METHODS

A friction measurement system was added to a multidirectional, established hip joint wear simulator, and its functionality was evaluated. In preliminary tests, a 50 mm diameter metal-on-metal prosthesis was tested in an optimal acetabular cup inclination angle (48°) and in a steep angle (70°) using a normal peak load (2 kN) and an increased peak load (3 kN). The test length was 100 h. Long-term adverse condition tests of 3 million cycles were run for three 52 mm metal-on-metal prostheses. The lubricant was diluted calf serum at 37 °C.

FINDINGS

In the 100 h tests, metal-on-metal frictional torque was not highly sensitive to the angle, load and serum degradation, and it was close to that of a conventional 28 mm prosthesis with a polyethylene cup, mostly below 5 Nm. However, a manyfold higher frictional torque (10 to 20 Nm) was observed in long-term metal-on-metal tests with substantial wear.

INTERPRETATION

To obtain a realistic prediction of the frictional behaviour of a hip design, long-term, multidirectional wear tests are necessary. The friction should preferably be measured during the wear test. In addition to normal conditions, adverse condition testing is strongly recommended.

Influence of joint kinematics on polyethylene wear in anatomic shoulder joint arthroplasty

BACKGROUND

Despite the positive results in total shoulder arthroplasties (TSAs), a higher revision rate is documented compared with total hip and knee replacements. Wear is the possible main cause of TSA failure in the long-term. This study investigated the effect of joint kinematics and the influence of the rotator cuff on the polyethylene wear performance in an anatomic TSA.

METHODS

Lifting a load of 2 kg with an abduction/adduction of 0° to 90° was simulated for 2 × 10⁶ cycles as a primary motion using a fully kinematic joint simulator. A combined rotation in anteversion-retroversion of ±5° and ±10° was also simulated. The force in the superior-inferior direction and the axial joint compression were applied under force control based on in vivo data of the shoulder. A soft tissue restraint model was used to simulate an intact and an insufficient rotator cuff.

RESULTS

The highest wear rate in the intact rotator cuff group was 58.90 ± 1.20 mg/10⁶ cycles with a combined rotation of ±10°. When an insufficient rotator cuff was simulated, the highest polyethylene wear rate determined was 79.67 ± 4.18 mg/10⁶ cycles.

CONCLUSIONS

This study confirms a high dependency of the polyethylene wear behavior and dimension on the joint kinematics in total shoulder replacement. This can be explained by an increasing cross-shear stress on the polyethylene component. The results obtained indicate that additional combined kinematics are an indispensable part of wear tests on anatomic shoulder replacements.

Fixed Bearing Knee Congruency – Influence on Contact Mechanics, Abrasive Wear and Kinematics

The objective of our study was to evaluate the in vitro wear behavior of fixed bearing designs for total knee arthroplasty in relation to contact mechanics and resultant kinematics for different degrees of congruency.

A finite element model was created for three knee articulations with increasing degrees of tibio-femoral congruency (flat, curved, and dished design). For the three different knee design configurations, in vitro wear simulation was performed according to ISO 14243–1.

Contact areas increased with increasing knee congruency, whereas the peak surface contact stresses decreased. The wear rates for the knee design configurations differed substantially between the three test groups (flat, curved, and dished).

Our observations demonstrate that increased congruency in conjunction with decreased surface contact stresses significantly contributes to reducing wear in fixed bearing knee articulations.

An Improved Tibial Force Sensor to Compute Contact Forces and Contact Locations In Vitro After Total Knee Arthroplasty

Contact force imbalance and contact kinematics (i.e., motion of the contact location in each compartment during flexion) of the tibiofemoral joint are both important predictors of a patient's outcome following total knee arthroplasty (TKA). Previous tibial force sensors have limitations in that they either did not determine contact forces and contact locations independently in the medial and lateral compartments or only did so within restricted areas of the tibial insert, which prevented them from thoroughly evaluating contact force imbalance and contact kinematics in vitro. Accordingly, the primary objective of this study was to present the design and verification of an improved tibial force sensor which overcomes these limitations. The improved tibial force sensor consists of a modified tibial baseplate which houses independent medial and lateral arrays of three custom tension-compression transducers each. This sensor is interchangeable with a standard tibial component because it accommodates tibial articular surface inserts with a range of sizes and thicknesses. This sensor was verified by applying known loads at known locations over the entire surface of the tibial insert to determine the errors in the computed contact force and contact location in each compartment. The root-mean-square errors (RMSEs) in contact force are ≤ 6.1 N which is 1.4% of the 450 N full-scale output. The RMSEs in contact location are ≤ 1.6 mm. This improved tibial force sensor overcomes the limitations of the previous sensors and therefore should be useful for in vitro evaluation of new alignment goals, new surgical techniques, and new component designs in TKA.

Engineering of a multi-station shoulder simulator

This work aimed to engineer a multi-station shoulder simulator in order to wear test shoulder prostheses using recognized shoulder activities of daily living. A bespoke simulator was designed, built and subject to commissioning trials before a first wear test was conducted. Five JRI Orthopaedics Reverse Shoulder VAIOS 42 mm prostheses were tested for 2.0 million cycles and a mean wear rate and standard deviation of 14.2 ± 2.1 mm(3)/10(6) cycles measured for the polymeric glenoid components. This result when adjusted for prostheses diameters and test conditions showed excellent agreement with results from hip simulator studies of similar materials in a lubricant of bovine serum. The Newcastle Shoulder Simulator is the first multi-station shoulder simulator capable of applying physiological motion and loading for typical activities of daily living.

The investigation of nanotribology of UHMWPE in fluid using atomic force microscopy

The fundamental understanding of the nanowear behavior of ultrahigh molecular weight polyethylene (UHMWPE) at a nanometer scale needs to be achieved to provide a better understanding of the initiating wear process and the potential causes of the wear particles generation of joint replacement. A nanotribology study was performed using atomic force microscope (AFM) tips sliding against UHMWPE surfaces in both water and bovine serum lubricants. Frictional properties of the nanocontact, and the geometry and mechanical features of the resulting scratches have been quantitatively characterized using AFM lateral force and PeakForce QNM modes. The results in this work indicated that the friction force and friction coefficient were smaller in serum lubricant than that in water. A normal load of 120 nN was the transition point for the plastic deformation of the material. The plastic deformation and material accumulation evolute with the increase of applied normal loads. Material pileup formed at the edges of the scratch, but they were not symmetrical due to the asymmetrical geometry of the silicon AFM tip. The height of the material pileup on the right side was approximately 40-70% of the pileup on the left side. The information may be useful for developing strategies for surface finishing techniques, which can control and minimize the production of asymmetric asperity and the resulting pileup with particular features. Furthermore, the moduli of the pileups were much larger than that of the fresh UHMWPE, which had the moduli greater than those of the inner scratch area. This suggested that stress concentration at these points could cause the pileup to be more susceptible to further wear processes, and eventually result in detaching from the bulk material.

Toward the interpretation of the combined effect of size and body weight on the tribological performance of total knee prostheses

PURPOSE

The research questions of the present study were: (1) Is total knee prosthesis wear behaviour influenced by implant size, body weight and their combined effect? (2) Are these findings significant and helpful from a clinical point of view?

METHODS

Two very different sizes of the same total knee prosthesis (TKP), previously tested with ISO 14243 parameters, were tested on a knee simulator for a further two million cycles using a modified ISO 14243 load waveform. Roughness examination was performed on the metallic components. Gravimetric and micro-Raman spectroscopic analyses were carried out on the polyethylene inserts.

RESULTS

The average volumetric mass loss was 69 ± 3 mm(3) and 88 ± 4 mm(3) for smaller and bigger size, respectively. Bigger TKPs are little influenced by an increased load, while the wear trend of the smaller TKP showed a redoubled slope, and more significant morphology changes were observed. However, the two sizes seem to behave similarly when subjected to a load increase of 15 %; the slope of the volumetric mass loss trend was comparable for the two sets of inserts, which did not appear significantly different also at the molecular level. Roughness average parameters of the lateral femoral condyle support this evidence.

CONCLUSIONS

It can be asserted that the body weight and implant size are relevant to the understanding of TKP wear behaviour. A post-implantation body weight increase in a patient with smaller knee dimensions could results in more critical effects on prosthesis long-term performance.

Vitamin-E blended and infused highly cross-linked polyethylene for total hip arthroplasty: a comparison of three-dimensional crystalline morphology and strain recovery behavior

Vitamin-E (α-tocopherol) is now recognized worldwide as one of the most promising antioxidant agents for highly cross-linked polyethylene (HXLPE) used in total joint replacements. In the contemporary manufacturing processes, two alternative methods are currently accepted to incorporate this antioxidant into polyethylene microstructure: (i) blending vitamin-E before consolidation and radiation crosslinking; (ii) infusing vitamin-E via a homogenizing heat treatment after radiation crosslinking. However, the effects of these technological differences on crystalline morphology and mechanical behavior of polyethylene remains to be fully elucidated. The aim of this paper is to quantitatively evaluate the microstructural differences of commercially available vitamin-E blended and infused HXLPE liner (referred to as Liner BL and IF, respectively). For this purpose, confocal/polarized Raman spectroscopy was used to systematically examine the three-phase percentages (amorphous (αa), crystalline (αc), and intermediate third phase (αt)), preferential molecular orientation (θp), and degree of crystalline anisotropy (〈P2(cosβ)〉). Additionally, we compared the time-dependent deformation of Liner BL and IF as obtained by uniaxial stress relaxation tests followed by strain recovery. Distinctive features of the near-surface αc, θp, and〈P2(cosβ)〉 were clearly observed within the first 35μm in the two studied liners. Despite the equivalent level of the bulk αc and 〈P2(cosβ)〉, higher restoring force against a uniaxial strain was observed in Liner IF, which reflects a higher crosslink density in its amorphous phase. On the other hands, a higher degree of surface orientational randomness was detected in Liner BL, which is structurally more beneficial for minimizing the in-vivo occurrence of strain-softening-assisted wear.

Raman spectroscopic study of remelting and annealing-induced effects on microstructure and compressive deformation behavior of highly crosslinked UHMWPE for total hip arthroplasty

Three-dimensional crystallographic morphologies were studied by means of confocal/polarized Raman spectroscopy as developed upon manufacturing in three different types of first and second generation highly crosslinked UHMWPE (HXLPE) acetabular liners. The impact of such microstructural characteristics on the deformation behavior of the liners was also evaluated and discussed from the viewpoint of molecular chain mobility. All the investigated liners showed similar microstructural transitions within the first 35 μm below their surfaces in terms of crystallinity, molecular orientation, and crystalline anisotropy. Interestingly, different postirradiation heat treatments (remelting or annealing in single step or in sequential steps) led to clear differences in the subsurface microstructure among the three liners. Remelted liner possessed both lower bulk crystallinity and degree of molecular orientation as compared to the annealed liners. Sequentially, irradiated/annealed liner showed the highest degree of crystallinity and orientation among the studied liners. The peculiar microstructure of this latter liner exhibited the highest restoring (shape-recovery) force against the applied uniaxial strain. Accordingly, the present study suggests that the sequential irradiation and annealing offers an efficient way to obtain microstructure quite suitable for attaining high creep resistance. However, all the investigated liners exhibited the significantly low values of surface anisotropy, which could be equally efficient in minimizing strain-softening-assisted wear phenomena.

Characterization of network parameters for UHMWPE by plane strain compression

Ultra-high molecular weight polyethylene (PE) is used as a bearing material for total joint replacement prostheses since it is a tough, wear-resistant semicrystalline polymer. Despite its high resistance to wear, PE components have shown measureable wear in vivo, which can cause wear-particle induced osteolysis. Crosslinking of PE using ionizing radiation has been shown to increase wear resistance since both chemical crosslinks and physical entanglements provide high resistance to wear. Molecular characterization of crosslinked PEs is usually conducted using equilibrium swelling or by quantifying gel content. In this study, we compared crosslink densities and molecular weight between crosslinks derived from equilibrium swelling to those obtained by applying the Gaussian and Eight-Chain model to describe plane strain compression of the PE melt. The latter approach has the advantage of accounting for contributions of entanglements to the overall crosslink density, which solvent-based techniques largely neglect. As expected, the crosslink density calculated from model fitting increased monotonically with increase in radiation dose in a 0-200kGy dose range, with a corresponding monotonic decrease in molecular weight between crosslinks, but provided higher values of crosslink density and correspondingly lower values of molecular weight between crosslinks compared to the equilibrium swelling technique.

Biotribology of a new bearing material combination in a rotating hinge knee articulation

The objective of the present study was to evaluate the biotribological behaviour, in terms of wear and particle release, of bushings and flanges made of carbon fibre reinforced poly-ether-ether-ketone (CFR-PEEK) in articulation with a zirconium nitride (ZrN) multilayer surface coating in a rotating hinge knee system. For the bushings of the rotational and flexion axles and the medial and lateral flanges, a CFR-PEEK with 30% polyacrylonitrile fibre content was used in a new bearing combination with ZrN. In vitro wear simulation was performed for patients with metal ion hypersensitivity, using a new rotating hinge knee design with a ZrN surface articulation in comparison with the clinically established cobalt-chromium version. For the bushings and flanges made of CFR-PEEK subjected to wear simulation, the volumetric wear rates were 2.3±0.48mm(3)million(-1) cycles in articulation to cobalt-chromium as reference and 0.21±0.02mm(3)million(-1) cycles in the coupling with ZrN, a 10.9-fold decrease. The released CFR-PEEK particles were comparable in size and shape for the coupling to cobalt-chromium and ZrN with most of the particles in a size range between 0.1 and 2μm. The study reveals comparable low wear and no macroscopic surface fatigue in a new rotating hinge knee design with highly congruent flanges and axles bushings made of CFR-PEEK articulating to a ZrN multilayer surface coating. Favourable wear behaviour of the newly introduced CFR-PEEK/ZrN coupling in comparison with the clinically established CFR-PEEK/cobalt-chromium articulation was found.

Advances in tribological testing of artificial joint biomaterials using multidirectional pin-on-disk testers

The introduction of numerous formulations of Ultra-high molecular weight polyethylene (UHMWPE), which is widely used as a bearing material in orthopedic implants, necessitated screening of bearing couples to identify promising iterations for expensive joint simulations. Pin-on-disk (POD) testers capable of multidirectional sliding can correctly rank formulations of UHMWPE with respect to their predictive in vivo wear behavior. However, there are still uncertainties regarding POD test parameters for facilitating clinically relevant wear mechanisms of UHMWPE. Studies on the development of POD testing were briefly summarized. We systematically reviewed wear rate data of UHMWPE generated by POD testers. To determine if POD testing was capable of correctly ranking bearings and if test parameters outlined in ASTM F732 enabled differentiation between wear behavior of various formulations, mean wear rates of non-irradiated, conventional (25-50kGy) and highly crosslinked (≥90kGy) UHMWPE were grouped and compared. The mean wear rates of non-irradiated, conventional and highly crosslinked UHMWPEs were 7.03, 5.39 and 0.67mm(3)/MC. Based on studies that complied with the guidelines of ASTM F732, the mean wear rates of non-irradiated, conventional and highly crosslinked UHMWPEs were 0.32, 0.21 and 0.04mm(3)/km, respectively. In both sets of results, the mean wear rate of highly crosslinked UHMPWE was smaller than both conventional and non-irradiated UHMWPEs (p<0.05). Thus, POD testers can compare highly crosslinked and conventional UHMWPEs despite different test parameters. Narrowing the allowable range for standardized test parameters could improve sensitivity of multi-axial testers in correctly ranking materials.

Comparative wear tests of ultra-high molecular weight polyethylene and cross-linked polyethylene

Wear particle-induced osteolysis is a major concern in hip implant failure. Therefore, recent research work has focussed on wear-resistant materials, one of the most important of which is cross-linked polyethylene. In view of this, the objective of this study was to compare the in vitro wear performance of cross-linked polyethylene to traditional ultra-high molecular weight polyethylene. In order to mimic appropriate in vivo conditions, a novel high-capacity wear tester called a circularly translating pin-on-disc was used. The results of this in vitro study demonstrated that the wear rate for cross-linked polyethylene was about 80% lower than that of conventional ultra-high molecular weight polyethylene. This difference closely matches in vivo results reported in the literature for total hip replacements that use the two biopolymers. The in vitro results were also verified against ASTM F732-00 (standard test method for wear testing of polymeric materials for use in total joint prostheses). The 50-station circularly translating pin-on-disc proved to be a reliable device for in vitro wear studies of orthopaedic biopolymers.

Clinical trade-offs in cross-linked ultrahigh-molecular-weight polyethylene used in total joint arthroplasty

Highly cross-linked formulations of ultrahigh-molecular-weight polyethylene (XLPE) offer exceptional wear resistance for total joint arthroplasty but are offset with a reduction in postyield and fatigue fracture properties in comparison to conventional ultrahigh-molecular-weight polyethylene (UHMWPE). Oxidation resistance is also an important property for the longevity of total joint replacements (TJRs) as formulations of UHMWPE or XLPE utilizing radiation methods are susceptible to free radical generation and subsequent embrittlement. The balance of oxidation, wear, and fracture properties is an enduring concern for orthopedic polymers used as the bearing surface in total joint arthroplasty. Optimization of material properties is further challenged in designs that make use of locking mechanisms, notches, or other stress concentrations that can render the polymer susceptible to fracture due to elevated local stresses. Clinical complications involving impingements, dislocations, or other biomechanical overloads can exacerbate stresses and negate benefits of improved wear resistance provided by XLPE. This work examines trade-offs that factor into the use of XLPE in TJR implants.

Effect of anterior-posterior and internal-external motion restraint during knee wear simulation on a posterior stabilised knee design

The objective of our study was to examine the effect of biphaseal AP translation and IE rotation restraint, using a system defined specifically for posterior stabilised knee designs, on wear, kinematics and particle release in comparison to linear motion restraint as required by the established ISO 14243-1:2002(E) protocol. In the ISOlinear groups, an AP motion restraint of 30 N/mm and an IE rotation restraint of 0.6 Nm/° were applied in the knee wear simulation. In the ISOgap biphaseal groups with PCL sacrificing implants, the restraining AP force was zero in a ±2.5mm range with, externally, a constant of 9.3N/mm applied proportionally to the AP translation of the tibia plateau, whereas the restraining IE torque was zero in a ±6° range with, externally, a constant of 0.13 Nm/° applied proportionally to the IE rotation of the tibia plateau. Using the ISOgap biphaseal protocol on a posterior stabilised knee design, we found an increase of 41% in AP translation and of 131% in IE rotation, resulting in a 3.2-fold higher wear rate compared to the results obtained using the ISOlinear protocol. Changes in AP translation and IE rotation ligament motion restraints have a high impact on knee joint kinematics and wear behaviour of a fixed bearing posterior stabilised knee design.

Surface damage versus tibial polyethylene insert conformity: a retrieval study

BACKGROUND

Surface damage of the tibial polyethylene insert in TKA is thought to diminish with increasing conformity, based on computed lower contact stresses. Added constraint from higher conformity may, however, result in greater forces in vivo.

QUESTIONS/PURPOSES

We therefore determined whether increased conformity was associated with increased surface pitting, delamination, creep, and polishing in a group of retrieved tibial inserts.

METHODS

We compared 38 inserts with a dished articular surface (conforming group) with 31 inserts that were unconstrained and nonconforming in the sagittal plane (less conforming group). The two groups had identical polyethylene composition and processing history. The articulating surfaces were scored for pitting, delamination, deformation/creep, and polishing. Evidence of edge loading and the presence of embedded bone cement were also recorded.

RESULTS

The conforming inserts were associated with higher delamination and pitting scores but lower polishing scores, even after adjusting for the effects of sex, age, insert thickness, and implantation duration. Long implantation duration and male sex were also associated with increased delamination, pitting, and polishing, whereas long shelf life was associated only with increased delamination. The conforming group also had approximately a fourfold greater prevalence of edge loading and approximately a threefold greater prevalence of embedded bone cement. The latter was associated with higher scores and proportions of delamination and pitting.

CONCLUSIONS

These findings suggest more conformity may increase surface fatigue damage in TKA. Higher constraint-induced stresses during secondary motions and more possibility for edge loading and bone cement capture on a dished surface may account for these results.

CLINICAL RELEVANCE

The selection of materials with high fatigue resistance may be particularly important for high-conformity/constraint tibial inserts. In addition, awareness of the benefits and trade-offs with conformity may allow better matching of TKA design to patient.

A COMPARATIVE STUDY ON WEAR BEHAVIOR OF HIP PROSTHESIS BY FINITE ELEMENT SIMULATION

A numerical approach was proposed to investigate the wear behavior occurred in the artificial hip joints in this paper. In the numerical simulations, the wear coefficients taken from pin-on-disk tests were introduced into the wear analysis model to assess the wear rates of polyethylene acetabular cups against metallic or ceramic femoral heads. For the established material combinations, different values of polyethylene wear rates were obtained respectively, which were not necessarily the realistic one as expected in vivo but could be confirmed after further discussion on the wear mechanism involved in wear tests. Current results indicated that the polyethylene/ceramic couples represented better wear performances than the polyethylene/metal couples. Furthermore, the ratio of wear rates for polyethylene cups against alumina and the metallic femoral heads was 0.5, which agreed well with that deduced from clinical studies or laboratory hip simulators. It is obvious that these comparable wear behaviors observed from clinics or laboratory studies also can be found by means of the numerical simulation.

Three-dimensional kinematics during deep-flexion kneeling in mobile-bearing total knee arthroplasty

We performed an in vivo radiographic analysis of tibiofemoral and polyethylene (PE) insert motions during weight-bearing kneeling beyond 120° of flexion in one high-flexion knee arthroplasty design to determine if kinematics changed over time and if axial rotation occur between the PE insert and the tibial baseplate. Twenty knees implanted with a posterior-stabilized rotating-platform (RP) knee arthroplasty were postoperatively evaluated at 3, 6, and 12 months. The averaged flexion angles were 122°, 129°, and 131° at 3, 6, and 12 months, respectively, showing that the improvement of flexion was achieved up to 6 months. The femoral condyles translated posteriorly from extension to maximum flexion. There was a significant increase in AP translation of femoral lateral condyle in the maximum flexion kneeling between 12 months and the two other intervals (p=0.0003 at 3 months and p=0.016 at 6 months), while no differences in those of medial condyle between all intervals. Almost all rotation occurred at the surface between the tibial baseplate and the PE insert (p=0.0479 at 3 months, p=0.0008 at 6 months, and p=0.0479 at 12 months), almost no rotation occurred at the surface between the PE insert and the femoral component. There were significant increases in the amount of internal rotation angle during full flexion between the tibial component and the PE insert up to 12 months. Knees implanted with this RP knee arthroplasty design show deep-flexion knee kinematics that are consistent with the implant design intent.

Mobile or fixed unicompartmental knee prostheses? In-vitro wear assessments to solve this dilemma

The unicompartmental knee prosthesis is an attractive alternative to total knee arthroplasty. Current UKP devices can be subdivided into two groups based on different design principles: fixed bearing knees, where the ultra-high molecular weight polyethylene meniscal component snap or press fits into the tibial tray, and mobile bearing designs which facilitate movement of the insert relative to the tray. The present study was aimed at comparing the in-vitro wear behaviour of fixed and mobile unicompartmental knee menisci under two configurations: the femoral components were cemented into a custom-made metallic block or, as a novelty of the present study, into a synthetic femur (i.e. under conditions which should better reproduce the in-vivo behaviour). Analyses were performed using a displacement-control knee wear simulator with "three-plus-one" stations. All the kinematics tests were set in accordance with the ISO 14243-1,2,3. Fixed and mobile polyethylene menisci showed a different wear behaviour: the fixation-frame influenced directional load transfer through each component in a qualitative and quantitative way. In fact, gravimetric results showed that under the metal block holder fixation, mobile components worn more than fixed components (weight losses of 8.7±2.0 mg and 2.6±1.09 mg, respectively); on the other hand, under the synthetic femur configuration, differences in wear behaviour were less pronounced and mobile menisci underwent a slightly lower weight loss than fixed components (4.5±2.2 mg vs. 6.7±1.4 mg). This different trend was explained in relation to the kinematic schemes of the two fixation methods. Raman spectroscopy, used to evaluate the UHMWPE crystallinity changes induced by mechanical stress, showed that mobile menisci specimens were more affected than the fixed components in both their superior and inferior surfaces, independent of the fixation-frame. In conclusion, if tested under conditions which should better reproduce the in-vivo behaviour, mobile UKPs did not show a worse wear behaviour than fixed components in terms of weight losses, although UHMWPE changes at the molecular scale could be detrimental.

An in vitro biotribological assessment of NUBAC, a polyetheretherketone-on-polyetheretherketone articulating nucleus replacement device: methodology and results from a series of wear tests using different motion profiles, test frequencies, and environmental conditions

STUDY DESIGN

In vitro biotribological investigation.

OBJECTIVE

To evaluate the wear resistance and long-term biodurability of NUBAC, a PEEK-on-PEEK articulating nucleus replacement device, using a series of wear tests with different motion profiles, test frequencies, and environmental conditions.

SUMMARY OF BACKGROUND DATA

Wear resistance is critical for any disc arthroplasty device, and osteolysis remains a clinical concern. The use of PEEK for an articulating load bearing nucleus replacement device represents a unique application of this material. NUBAC has an inner ball/socket articulation for motion, similar to total disc replacements. American Society for Testing and Materials and International Organization for Standardization have recommended wear testing methodologies for total disc replacements, however, they have not been clinically validated. Therefore, a series of wear tests were performed to characterize the wear properties of the device.

METHODS

Four groups of devices were evaluated. Group 1 consisted of +/-7.5 degrees flexion/extension to 10 million cycles (Mc) followed by +/-7.5 degrees lateral bending to 10 Mc, alternated to 40 Mc. Groups 2 to 4 consisted of International Organization for Standardization motion and load profiles to 10 Mc, except Group 3 incorporated frequency shifting to ensure a nonrepetitive load and motion profile. Group 4 underwent simulated aging. All studies incorporated a load magnitude of 225 to 1024 N. The average wear rates were determined using linear regression analysis with significant differences between groups determined (analysis of variance).

RESULTS

A wear-in period was observed from 0 to 1 Mc. Wear rates were therefore calculated from 1 Mc. The wear rate for Group 1 was significantly less than Groups 2 to 4 through 10 Mc. From 1 to 5 Mc, the wear rate for Group 1 was significantly less than all groups, with Groups 2 to 4 not significantly different from each other. The wear rates for Groups 2 to 4 were seen to decrease after 5 Mc with only Group 3 significantly different than Group 1. The Group 1 wear rate was consistent throughout the test duration of 40 Mc.

CONCLUSION

The experimental wear rates compare well with the reported wear rates of other material combinations used in nucleus replacement and total disc arthroplasty. Overall, wear rates were relatively low and consistent, suggesting long-term durability, a critical requirement of disc arthroplasty devices.

Potential for adhesive wear in friction couples of UHMWPE running against oxidized zirconium, titanium nitride coatings, and cobalt-chromium alloys

The classical wear mechanisms abrasion, fatigue, and adhesion are the most frequent causes of surface changes of ultra high molecular weight polyethylene (UHMWPE) in artificial joints. The counterpart material has a strong influence on the wear and friction behavior of artificial joints due to its abrasive properties and adhesive interaction with UHMWPE. The formation of a transfer layer on the counterpart in UHMWPE bearing systems is often described as being a clear indication of strong adhesive forces. The influence of using a cobalt-chromium-molybdenum (CoCrMo) alloy, a titanium nitride plasma coating or an oxidized zirconium alloy on adhesive wear was studied. The surface free energy and the bonding forces of these counterpart materials to UHMWPE were investigated. Catalytic effects on the degradation behavior of polyethylene, the micro friction behavior, and the build-up and constitution of a transfer layer deposited under loads, and relative velocities that are relevant in knee joints were analyzed.