The modification of bearing surfaces with poly(2-methacryloyloxyethyl phosphorylcholine) (PMPC) is known to increase the hydration of the surfaces and decrease the wear of the substrates. PMPC grafting to acetabular liner of total hip arthroplasty showed a drastic reduction of cross-linked polyethylene (CLPE) wear in a long-term hip simulator test and achieved a good short-term clinical result. To apply this technique to other joint prostheses, the wear resistance under various conditions needs to be evaluated because every joint has a different wear mode. ASTM F732 gives a method that disk shaped polymer specimen is loaded with hemispherical pin using pin-on-disk tester, which is suitable for hydrated polymer because the lubricant is supplied every loading cycle on the surface. The purpose of this study is to evaluate the performance of PMPC-grafted hydrated CLPE under multidirectional wear condition in anticipation of applying PMPC to various prostheses. The CLPE disks of 3 or 6-mm in thickness were machined from a bar stock. The PMPC was grafted onto the CLPE surfaces using a photoinduced polymerization of MPC in aqueous medium. All disks were irradiated with a total amount of 75-kGy gamma-ray. The wear resistance of the CLPE and PMPC-grafted CLPE disks against Co-Cr-Mo alloy pin was evaluated using Ortho-POD pin-on-disk tester. The disks were fixed to the tester with a Ti-6Al-4V alloy plate that has screw hole in the center. The test conditions were a static load of 213 N, sliding shape of 5 mm × 10 mm rectangular, frequency of 1 Hz and maximum cycles of 1.0 × 10. 6. [Fig. 1]. Gravimetric wear was determined by weighing the disks and soak controls were used to compensate for the fluid absorption. After the wear test, volumetric changes of sliding and backside surfaces of disks were evaluated using a noncontact optical three-dimensional profiler. The PMPC-grafted surface showed decrease in the gravimetric wear drastically [Fig. 2]. The thickness of CLPE had no substantial effect on the wear resistance. Three-dimensional profile measurements of sliding surfaces detected a substantial volumetric penetration; the corner of sliding track were deeper than the straight-line portion. Backside extrusion was observed in all disks. The thickness of CLPE affected both volumetric penetration and backside extrusion for both untreated and PMPC-grafted CLPE. The PMPC grafting had no discernible effect on volumetric changes [Fig. 3]. Results of this study revealed: (1) the PMPC-grafted surface decreases wear of CLPE, however, the thickness of disk has no effect, in contrast, (2) thinner thickness of CLPE increases the volumetric changes including penetration in sliding surface and extrusion in back surface but the PMPC-grafted surface has no effect. Gravimetric wear did not correlate with the volumetric penetration in sliding surface because the volumetric penetration might be caused by not only the wear but also the creep deformation. In conclusion, hydrated bearing surface and thickness of bearing substrate are essential for the wear and fatigue resistance properties for an increasing longevity of artificial joint. In addition, PMPC grafting is a promising technique for increasing the longevity of various joint prostheses

The main objective of joint arthroplasty is to improve activities of daily living of the patient. However, normal daily activities may lead to separation of articular surfaces of an artificial joint, possibly as a result of a combined impact and sliding motion. Therefore, the properties of articular surfaces define the durability of implant materials. Modification of bearing surfaces with poly(2-methacryloyloxyethyl phosphorylcholine) (PMPC) increases the hydration of the surfaces and decreases the wear of the substrates. Hence, a PMPC layer can potentially cushion the impact and improve the resistance of cross-linked polyethylene (CLPE). This study aimed to explore the fatigue and wear resistance of PMPC-grafted hydrated CLPE under impact-to-wear conditions using a pin-on-disk tester. The surfaces of a CLPE disk (3- or 6-mm thick) were modified with PMPC by photoinduced polymerization and were sterilized using gamma rays. The wear resistance of PMPC-grafted CLPE disks against a Co-Cr-Mo alloy pin was evaluated and compared to that of untreated disks. The disks were fixed to the tester with a metal plate (Ti-6Al-4V alloy) that had a central hole. The test was performed for 2 × 10. 6. cycles of repetitive impact and unidirectional sliding with the maximum load of 150 N, sliding distance of 10 mm, and frequency of 1 Hz [Fig. 1]. Gravimetric wear was determined by weighing the disks, and soak controls were used to compensate for fluid absorption. Volumetric changes in the surfaces of the disks were evaluated using a three-dimensional non-contact optical profiler. The average gravimetric wear (mg) after 2 × 10. 6. cycles was 0.000/0.120 for CLPE (3/6 mm) and −0.073/–0.137 for PMPC-CLPE (3/6 mm). The weight gain of the PMPC-CLPE disks was due to their greater fluid absorption compared to that of the soak controls under the impact-to-wear conditions, as judged from the fact that during the load-soak in the lubricant this gain was observed for all the disks irrespectively of PMPC grafting. PMPC-grafting decreased the gravimetric wear of CLPE (p < 0.01) in the 6-mm group, whereas the thickness of the CLPE disks had no substantial effect on the wear resistance [Fig. 2]. In all cases, three-dimensional measurements detected a remarkable volumetric penetration in the impact-sliding surfaces and an extrusion of CLPE from the backside surfaces into the hole in the metal plate. Both the volumetric penetration and backside extrusion were smaller in the 6-mm group. The PMPC grafting had no discernible effect on these volumetric changes [Fig. 3]. Even after 2 × 10. 6. cycles of impact loads, mechanical fracture or delamination of the impact-sliding or backside surfaces were hardly observed in all the groups. The results of this study revealed that: (1) PMPC-grafting of CLPE surfaces decreased the gravimetric wear irrespectively of the disk thickness; and (2) thinner CLPE increased the risk of volumetric changes, including penetration in the impact-sliding surface and extrusion of the backside surface. In conclusion, PMPC grafting can potentially improve the wear resistance of the bearing surface of biomaterials even under impact-to-wear conditions, increasing the longevity of artificial joints

Aims. Highly cross-linked polyethylene (HXLPE) greatly reduces wear in total hip arthroplasty, compared to conventional polyethylene (CPE). Cross-linking is commonly achieved by irradiation. This study aimed to compare the degree of cross-linking and in vitro wear rates across a cohort of retrieved and unused polyethylene cups/liners from various brands. Methods. Polyethylene acetabular cups/liners were collected at one centre from 1 April 2021 to 30 April 2022. The trans-vinylene index (TVI) and oxidation index (OI) were determined by Fourier-transform infrared spectrometry. Wear was measured using a pin-on-disk test. Results. A total of 47 specimens from ten brands were included. The TVI was independent of time in vivo. A linear correlation (R. 2. = 0.995) was observed between the old and current TVI standards, except for vitamin E-containing polyethylene. The absorbed irradiation dose calculated from the TVI corresponded to product specifications for all but two products. For one electron beam-irradiated HXLPE, a mean dose of 241% (SD 18%) of specifications was determined. For another, gamma-irradiated HXLPE, a mean 41% (SD 13%) of specifications was determined. Lower wear was observed for higher TVI. Conclusion. The TVI is a reliable measure of the absorbed irradiation dose and does not alter over time in vivo. The products of various brands differ by manufacturing details and consequently cross-linking characteristics. Absorption and penetration of electron radiation and gamma radiation differ, potentially leading to higher degrees of cross-linking for electron radiation. There is a non-linear, inverse correlation between TVI and in vitro wear. The wear resistance of the HXLPE with low TVI was reduced and more comparable to CPE. Cite this article: Bone Joint Res 2024;13(11):682–693

Cross-shear has been shown to increase ultra-high molecular weight polyethylene (UHMWPE) wear in pin-on-disk, total knee, total hip, and spinal disc replacement testing. Computer modelling of implant wear holds promise for improving efficiency in the development of new implant designs, but it is desirable to accurately account for the effects of cross-shear in the computational simulation. Several studies have sought to propose a quantitative metric for cross-shear in multidirectional sliding and to correlate average cross-shear intensity with apparent wear rate measured in experiments. The apparent wear rate accounts for the total volume loss from all points on the UHMWPE surface. In principle, if the cross-shear metric correlates with experimental wear rates, it is then possible to predict an estimated wear rate for any arbitrary set of kinematic inputs. UHMWPE wear may then be simulated numerically with some form of Archard’s law. One limitation of the above approach is that counter-face kinematics are homogenized by the use of a spatially and temporally averaged apparent wear rate. In a sliding contact interface of a joint implant in vivo, the intensity of cross-shear wear may vary with time and location on the surface. To address this variation we have proposed a novel cross-shear metric (x*) and developed a modified form of Archard’s law that is capable to differentiate between unidirectional and multidirectional sliding wear. The wear model and x* have been implemented in an explicit finite element framework (ABAQUS) that is capable of quantifying wear from any number of wear surfaces (e.g., front side, backside, post) with completely general geometry and loading conditions. Preliminary validation of x* and the wear model have been performed by comparison with data from the open literature. Cross-shear metric x* is easy to compute, exhibits invariance to the choice of kinematic reference frame, and is able to reliably distinguish between similarly shaped sliding paths of different lengths – all improvements compared to cross-shear metrics described elsewhere. The wear model that incorporates x* has shown good agreement with pin-on-disk and cervical disc replacement wear results previously reported. Ongoing research focuses on demonstrating similar validity of the model for cross-shear wear in hip and knee replacements

Introduction. Fretting crevice-corrosion (tribocorrosion) of metallic biomaterials is a major concern in orthopedic, spinal, dental and cardiovascular devices. 1. Stainless steel (i.e., 316L SS) is one alloy that sees extensive use in applications where fretting, crevices and corrosion may be present. While fretting-corrosion of this alloy has been somewhat studied, the concept of fretting-initiating crevice corrosion (FICC), where an initial fretting corrosion process leads to ongoing crevice-corrosion without continued fretting, is less understood. This study investigated the susceptibility of 316L SS to FICC and the role of applied potential on the process. The hypothesis is crevice-corrosion can be induced in 316L SS at potentials well below the pitting potential. Materials and Methods. A pin-on-disk fretting test system similar to that of Swaminathan et al. 2. was employed. Disks were ∼35 mm in diameter and the pin area was ∼500 mm. Samples were polished to 600 mm finish, cleaned with ethanol and distilled water. An Ag/AgCl wire as the reference, a carbon counter electrode and phosphate buffered saline (PBS, pH 7.4, Room T) were used for electrochemical testing. Load was controlled with a dead-weight system, monitored with a six-axis load cell (ATI Inc.). Interfacial motion was captured with a non-contact eddy current sensor (0.5 mm accuracy). Motion and load data acquisition was performed with Labview (National Instruments). Samples were loaded to ∼2 N. The potential per tests was increased from −250 to 250 mV (50 mV increments) with new locations and pins used in each repeat (n=3). Testing incorporated a 1 min rest before fretting (5 min, 1.25 Hz, 60 mm displacement saw tooth pattern). Fretting ceased and the load was held while currents were captured for another 5 min to assess ongoing crevice corrosion. Results. Testing showed that crevice corrosion can be initiated within minutes of fretting (or in a few cycles depending on potential; Fig. 1). Potentials as low as −100 mV showed evidence of corrosion, while sustained crevice corrosion was seen at −50 mV. As the potential increased above −50 mV, susceptibility to FICC increased. Fig. 2 is a typical cyclic polarization curve for 316L SS in PBS without fretting. Pitting starts at 400 mV vs Ag/AgCl, and the protection potential in this case is around potentials where FICC can be induced. Discussion. This study showed that 316L SS is prone to FICC starting at −100 mV and the severity of the crevice-corrosion damage depends on the applied potential (Fig. 3). Current after cessation of fretting takes longer to return to baseline or does not return indicating ongoing corrosion without fretting (Fig. 1). If the pin and disk are separated, the crevice-corrosion process stops immediately. The region immediately outside the fretting contact was crevice-like with a very small separation distance between the pin and disk surface which allowed crevice corrosion to develop (Fig. 3). Conclusion. 316L SS can undergo FICC at potentials close to normal physiological electrode potential conditions. Few fretting cycles are required to develop conditions for continued crevice-corrosion. Higher potentials increased the susceptibility of FICC in 316L SS

Generic walking profiles applied to mechanical knee simulators are the gold standard in wear testing of total knee replacements (TKRs). Recently, there was a change in the international standard (ISO) for knee wear testing (ISO 14243-3): the direction of motion in the anterior/posterior (AP) and internal/external (IE) directions were reversed. The effects of this change have not been investigated, therefore it is not known whether results generated by following this new standard can be compared to historical wear tests which used the old standard. Using a finite element analysis (FEA) model of a TKR in parallel with an energy based wear model and adaptive remeshing, we investigated differences in wear between the newest ISO standard developed in 2014, and the previous ISO standard developed in 2004. CAD models of a left sided NexGen Cruciate Retaining (CR) TKR (Zimmer, Warsaw, IN) were used to create the FEA model (Figure 1). The loads and motions specified by simulator standards ISO 14243-3(2004) and ISO 14243-3(2014) were applied to the model. Analyses were run using ABAQUS v6.13-2 Standard (Dassault Systèmes, Waltham, MA). 8 node hexahedral elements were used to model the UHMWPE component. The contact was modeled as penalty contact, with the friction coefficient set to 0.04 on the articular surface. The cobalt chromium molybdenum femoral component was modeled as a rigid surface, utilizing a mix of 2. nd. order quadrilaterals and tetrahedrons. Wear of the polyethylene (PE) component was predicted to 1,000,000 cycles using a previously published frictional energy-based wear model. The wear model, developed from data generated in wheel-on-flat tests, utilizes two parameters defining the frictional energy required to remove a unit volume of material both parallel (3.86E8 J/mm. 3. ) and perpendicular (3.55E7 J/mm. 3. ) to the primary polyethylene fibril direction. Primary fibril direction for the analysis was set to the AP direction. Wear for each simulation of a gait cycle was scaled to 500,000 cycles. Two gait cycles were simulated representing 1,000,000 cycles in total. Adaptive remeshing was driven by the wear model, with the mesh being updated every time increment to simulate material ablation. The time step size was variable with a maximum of 0.01s. The FEA predicted higher wear rates for the newest ISO standard (7.34mg/million cycles) compared to the previous standard (6.04mg/million cycles) (Figure 2). Comparing the predicted wear scars (Figure 3), the new version of the standard covered a larger percentage of the total articular surface, with wear being more spread out as opposed to localized. This is more similar to what is seen in patient retrievals. The results of the study suggest that major differences between the old and the new ISO standard exist and therefore historical wear results are not comparable to newly obtained results. In addition, this study demonstrates the utility of FEA in wear analysis, though the wear model needs further work and validation before it can be used as a supplement to simulator testing. Validation of the wear model against simulator tests and pin-on-disk experiments is currently underway. For figures/tables, please contact authors directly.

Background. Wear and fatigue damage to polyethylene components remain major factors leading to complications after total knee and unicompartmental arthroplasty. A number of wear simulations have been reported using mechanical test equipment as well as computer models. Computational models of knee wear have generally not replicated experimental wear under diverse conditions. This is partly because of the complexity of quantifying the effect of cross-shear at the articular interface and partly because the results of pin-on-disk experiments cannot be extrapolated to total knee arthroplasty wear. Our premise is that diverse experimental knee wear simulation studies are needed to generate validated computational models. We combined five experimental wear simulation studies to develop and validate a finite-element model that accurately predicted polyethylene wear in high and low crosslinked polyethylene, mobile and fixed bearing, and unicompartmental (UKA) and tricompartmental knee arthroplasty (TKA). Methods. Low crosslinked polyethylene (PE). A finite element analysis (FEA) of two different experimental wear simulations involving TKA components of low crosslinked polyethylene inserts, with two different loading patterns and knee kinematics conducted in an AMTI knee wear simulator: a low intensity and a high intensity. Wear coefficients incorporating contact pressure, sliding distance, and cross-shear were generated by inverse FEA using the experimentally measured volume of wear loss as the target outcome measure. The FE models and wear coefficients were validated by predicting wear in a mobile bearing UKA design. Highly crosslinked polyethylene (XLPE). Two FEA models were constructed involving TKA and UKA XLPE inserts with different loading patterns and knee kinematics conducted in an AMTI knee wear simulator. Wear coefficients were generated by inverse FEA. Results. Predicted wear rates were within 5% of experimental wear rates during validation tests. Unicompartmental mobile bearing back-side wear accounted for 46% of the total wear in the mobile bearing. Wear during the swing phase was 38% to 44% of total wear. Discussion & Conclusions. Crosslinking polyethylene primarily decreased (by nearly 10-fold) the wear generated by cross-shear. This result can be explained by the reduced propensity of crosslinked polyethylene molecules to orient in the dominant direction of sliding. A highly crosslinked fixed-bearing polyethylene insert can provide high wear performance without the increased risk for mobile bearing dislocation. Finite element analysis can be a robust and efficient method for predicting experimental wear. The value of this model is in rapidly conducting screening studies for design development, assessing the effect of varying patient activity, and assessing newer biomaterials. This FEA model was experimentally validated but requires clinical validation

Today's aging society is seeing an increase of patients with rheumatoid arthritis and osteoarthritis, as well as an increase in joint replacement surgery. The artificial joints used in this surgery frequently uses ultra-high molecular weight polyethylene (UHMWPE) as a bearing material. However, UHMWPE wear particles are considered to be a major factor in long-term osteolysis, and implant loosening. Many researchers have reported that the volume and size of particles are critical factors in macrophage activation, with particles in the size range of 0.1 – 1.0 μm being the most biologically active. The micro slurry-jet erosion (MSE) apparatus was introduced to minimize the amount of wear, and increase the size of UHMWPE wear particles by texturing the surfaces of Co-Cr-Mo alloy implants. The MSE apparatus uses a slurry of alumina particles (WA#8000: average diameter 1.2 μm) mixed with water. The slurry and compressed air are mixed within an injection nozzle, which is then applied to the Co-Cr-Mo alloy at high speed to achieve a desired nano-textured surface. In this study, four Co-Cr-Mo alloy surface profiles were prepared. The MSE injection nozzle was fed 40.0 mm in alternating directions across each surface with an orthogonal step of 0.5 mm. The surface M-1 was processed with an injection nozzle feed rate of 1.0 mm/s, and obtained a surface roughness of 5.7 nm. M-2 was processed with a feed rate of 2.0 mm/s, and had a surface roughness of 2.3 nm. The M-4 surface used a 40.0 mm alternating directions surface feed, but with a 1.0 mm orthogonal step, and an injection nozzle feed rate of 0.5 mm/s. It obtained a surface roughness of 4.0 nm. The G-1 surface, with a roughness of 10.0 nm, was processed with the typical lapping method, which is used in conventional artificial joints [Fig. 1]. A pin-on-disk wear tester, capable of multidirectional motion, was used to assess which surface was the most appropriate for artificial joints. The UHMWPE pins were flat ended cylinders, 12.0 mm in diameter, and were placed on the disk with a contact pressure of 6.0 MPa. Tests were carried out in 25% (v/v) fetal calf serum with sodium azide to retard bacterial growth. A sliding speed of 12.1 mm/s, and a total sliding distance of 15.0 km were applied. The wear weight of the MSE textured surface M-1 was significantly lower than the wear weight of the conventional surface. Moreover, the percentages of various wear particle sizes obtained from MSE surface texturing was significantly different from those obtained from the traditional surface

Introduction. Ultra high molecular weight polyethylene (UHMWPE) has been used successfully as a bearing material in hip, knee, and shoulder joint replacements. However, there are problems to cause a failure in UHMWPE component, which are wear behavior and creep deformation. Continuous bearing motion and dynamic load have occurred to UHMWPE wear debris caused osteolysis in periprosthetic tissue and to plastic deformation of joint component, and subsequent aseptic loosening of components. Therefore, many studies have being carried out in order to reduce wear debris and to improve mechanical strength from UHMWPE, and there is tremendous improvement of mechanical property in UHMWPE from gamma irradiated conventional UHMWPE (GIPE), highly crosslinked PE (XLPE), and XLPE with vitamin E1, 2. Friction has a significant one of the factors effect on the wear and creep deformation. In this study, the short-term frictional behaviors of three typical types of GIPE, remelted XLPE (R-XLPE), and s annealed XLPE (A-XLPE), and XLPE with Vitamin E against Co-Cr alloy were compared under three levels of contact pressures which occured in hip, knee, and shoulder joints. Methods. Friction tests were conducted with UHMWPE against Co-Cr alloy by using pin-on-disk type triboteter. For test, tribotester performed in a repeat pass rotational slidintg motion with a velocity of 60rpm. Applied contact pressure selected three kinds of levels, 5, 10, and 20MPa which were within the range of maximum contact pressures for total hip, knee, and shoulder joint replacements. To analyze the frictional effect of UHMWPE type, it conducted t-test and p-values less than 0.05 were used to determine the statistically significant difference. Results. In this study, it was observed that coefficients of friction (COF) were affected by various conditions, kinds of materials and applied load. We can reveal the frictional behavior of UHMWPE in various contact pressures. The average of the COF measured that GIPE was 0.029∼0.0423, R-XLPE was 0.018∼0.031, A-XLPE was 0.023∼0.038, and XLPE with Vitamin E was 0.013∼0.027 under 5, 10, and 20MPa. Discussion. COF of R-XLPE, A-XLPE, and XPLE with Vitamin E were lower than GIPE for all levels of contact pressures. This study showed the trend that COF decreased as contact pressure increased. Also, XPLE with Vitamin E has lowest frictional values among UHMWPEs. In the viewpoint of applied load, it was decreased as a contact pressure increased for COF of GIPE, RXLPE, and AXLPE against Co-Cr alloy. COF of GIPE, XLPEs, and XLPE with Vitamin E against Co-Cr alloy were as low as using bio materials compared with the COF of cartilage to cartilage, which was about 0.024. Conclusions. In conclusion, average COF of XLPE with Vitamin E was significantly lower than those of R-XLPE and A-XLPE. XLPEs showed much lower COF than GIPE

Introduction. First-generation annealed HXLPE has been clinically successful at reducing both clinical wear rates and the incidence of osteolysis in total hip arthroplasty. However, studies have observed oxidative and mechanical degradation occurring in annealed HXLPE. Thus, it is unclear whether the favorable clinical performance of 1st generation HXLPE is due to the preservation of bearing surface tribological properties or, at least partially, to the reduction in patient activity. The purpose of this study was to evaluate the in vitro wear performance (assessed using multidirectional pin-on-disk (POD) testing) of 1. st. -generation annealed HXLPE with respect to in vivo duration, clinical wear rates, oxidation, and mechanical properties. Materials and Methods. 103 1. st. -generation annealed HXLPE liners were collected at revision surgery. 39 annealed HXLPE liners were selected based on their implantation time and assigned to three equally sized cohorts (n=13 per group); short-term (1.4–2.7y), intermediate term (5.2–8.0y) and long-term (8.3–12.5y). From each retrieved liner, two 9-mm cores were obtained (one from the superior region and one from the inferior region). Sixteen cores were fabricated from unimplanted HXLPE liners that were removed from their packaging and six pins from unirradiated GUR 1050 resin served as positive controls. Multidirectional POD wear testing was conducted against wrought CoCr disks in a physiologically relevant lubricant (20 g/L protein concentration) using a 100-station SuperPOD (Phoenix Tribology, UK). Each pin had its own chamber with 15mL lubricant maintained at 37±1°C. An elliptical wear pattern with a static contact stress of 2.0 MPa was employed. Testing was carried out to 1.75 million cycles at 1.0 Hz and wear was assessed gravimetrically. POD wear rates were calculated using a linear regression of volumetric losses. In vivo penetration was measured directly using a calibrated micrometer. Oxidation was assessed on thin films obtained from superior and inferior regions of the liners (ASTM 2102). Mechanical properties were assessed using the small punch test (ASTM 2183). Results. In vitro wear rates from the SuperPOD were positively correlated with implantation time (Rho=0.27; p=0.015) and average oxidation (Rho=0.36; p=0.004) at the bearing surface of the retrieved HXLPE liners. All retrieved HXLPE cohorts had lower in vitro wear rates than uncrosslinked positive control (p≤0.03) and higher wear rates than the never-implanted HXLPE cohort (p <0.001). POD wear rates were negatively correlated with small punch ultimate load (p<0.01). However, the in vitro wear rates were not correlated with clinical penetration rates (p=0.71). Discussion. This study investigated the effects of in vivo degradation on 1. st. -generation annealed HXLPE liners. The data in this study suggest that the tribological properties degrade due to in vivo oxidation as the liner is exposed to the in vivo environment. The clinical implications of these findings, however, are not clear as the clinical penetration rates were not correlated with the in vitro POD wear rates. This may be partially due, to decreasing patient activity as they age. These findings will be useful for comparison for evaluating the in vitro wear properties of other HXLPEs, including 2nd generation HXLPE. Acknowledgements. This study was supported by the NIH(NIAMS) R01AR47904

It have been reported that the wear volume of vitamin E-containing UHMWPE tested with a knee joint simulator was approximately 30% lower than that of virgin UHMWPE at 5 million cycles. However, the wear resistance mechanism of vitamin E-containing UHMWPE has not yet been clarified. The present study examines the effects of the addition of vitamin E on the frictional properties of ultra-high molecular weight polyethylene (UHMWPE) under several different load and serum conditions. Friction tests were carried out using a computer-controlled pin-on-disk friction test apparatus. The UHMWPE pin was mounted vertically at the tip of the leaf spring and linear reciprocating sliding motion for 2,000 cycles with an amplitude of 1 mm and a frequency of 1 Hz, was applied under 3 MPa or 30 MPa loading against Co-28Cr-6Mo alloy disk. The lubricant bath was filled with 5 ml of ultrapure water, fresh serum, post-friction (PF) serum or diluted-PF (DPF) which were kept at a temperature of 37°C. The friction force between the UHMWPE pin and the Co-28Cr-6Mo alloy disk was calculated from the displacement of the leaf spring during the sliding motion. Vitamin E-containing UHMWPE showed a significantly higher friction force than that of virgin UHMWPE in fresh serum lubricant at 30 MPa loading, while there were little differences in either ultrapure water or PF serum or DPF serum. And vitamin E-containing UHMWPE tends to exhibit a lower dynamic friction force within the first few hundred cycles in the case of all serum lubricants at 30 MPa loading. These results suggest that some interaction between the UHMWPE surface and the native conformation proteins was specifically affected by the addition of vitamin E and that some weeping of vitamin E might occur at early stage of sliding. Our results also suggest the importance of the conformational changes of serum proteins for the wear testing

Statement of Purpose:. The wear rate of Ultra High Molecular Weight Polyethylene (UHMWPE) in joint replacements has been correlated to both contact area and contact stress in the literature, [1], [2]. In both publications and our experiment, UHMWPE articulated with a polished surface of cobalt-chromium alloy was evaluated using a Pin-On-Disk (POD) apparatus (AMTI) implementing bi-directional movement. In publication [1], volumetric wear was independent of normal load and dependent upon increasing contact area. The results demonstrated that increasing contact stress decreased wear rates twofold. In publication [2], at maximum cross-shear, wear was proportional to nominal contact area and wear factors normalized to area are more appropriate than load based wear factors. In both studies, the contact surface areas of the POD pins were reduced by decreasing the diameters of the POD Pins. In our experiment, the contact area was dependent on textured POD Pin 390 (T390) which had low wear [3]. T390 reduced the normal POD contact area from 71 mm. 2. to 8.26 mm. 2. Hydroxylapatite (HA) particles were introduced to the serum to simulate third body wear debris. We hypothesized that the normal POD Pins would have greater wear rates than the textured POD Pins. A measurement of 0.14 mg HA particles per 250 mL of serum was used for each test 0.33 million cycles. Methods:. The GUR 1020 resin XLK POD Pins were gamma irradiated to 50 kGy in a vacuum package and then remelted. Three (3) T390 POD pins and nine (9) untextured XLK POD Pins were used. Three untextured XLK POD Pins were tested against three T390 POD pins. The other six (6) untextured XLK POD Pins were used as soak controls. Each pin articulated against a polished, high carbon wrought CoCr metal alloy counterface (ASTM F1537; diameter = 38.1 mm; thickness = 12.7 mm). Wear rate tests were for 1.98 million cycles. In order to perform the t-test analysis, the wear rates for each pin were given by the slope of the linear regression line through the individual data points (cycle count, cumulative wear), excluding the (0, 0) point. Results:. The probability for the means between the T390 POD pins and the untextured XLK POD Pins was *p = 0.009. T390 wear rates were statistically significant as compared to the untextured XLK POD Pin wear rates. The T390 POD Pin is illustrated in Figure 1. Figures 2 and 3 summarize the wear rates between T390 POD Pins and the untextured POD Pins with and without HA particles. Conclusions:. The wear rates between T390 and untextured POD pins did not take into account that the POD pins were not cleaned using a solution to remove potentially embedded HA particles. The follow-on experiment will use a special cleaning method to remove all HA particles after each test cycle

Pin-on-disk studies have demonstrated the role that cross-shear plays in polyethylene wear. It has been found that applying shear stresses on the polyethylene surface in multiple directions will increase wear rates significantly compared to linear sliding. Hip and knee joint replacements utilize polyethylene as a bearing surface and are subjected to cross-shear motions to various degrees. This is the mechanism that produces wear particles in hip and knee arthroplasty bearings and if excessive may lead to osteolysis, implant loosening, and failure. The amount of cross-shear is dependent on the bearing diameter and the angular motion exerted onto the bearing due to the gait of the patient. This study will determine the effect of sliding curvature (angular change per linear sliding distance) on the wear rate of polyethylene. Virgin polyethylene blocks were machined with a 28mm diameter bearing surface and against 28mm cobalt chromium femoral heads in a hip simulator. Dynamic loading was applied simulating walking gait but the motion differed between testing groups. Typical walking gait testing utilizes 23° biaxial rocking motion, in this study, 10°, 15°, 20°, and 23° biaxial rocking motions resulting in various sliding curvatures. Sliding motion path is described in Figure 1 and is a function of the bearing radius and the rocking angle. With increased rocking angle, the sliding distance reduces per cycle and the sliding path becomes more curved (more angular change per linear distance of sliding). Despite a significant increase in sliding distance at higher rocking angles, wear rates were relatively unchanged and ranged from 57mm3/mc to 62mm3/mc. Wear rates per millimeter increased exponentially with reduced sliding arc radius (smaller rocking angle) as shown in Figure 2. This study suggests that wear of polyethylene is highly dependent on sliding path curvature. The sliding path is largely a function of the bearing diameter and the patient activity. Large bearing diameter implants have been recently introduced to increase joint stability. Sliding distance increases proportional to the bearing radius which has led to some concerns regarding increased wear in larger bearings. However, in vitro wear studies have not shown this trend. Increased bearing diameter also increases the sliding path curvature which this study has shown to cause a reduction in wear roughly proportional to the radius of the bearing. Therefore, the increase in wear due to sliding distance is offset by the reduction in wear caused by the sliding curvature resulting in no significant change in wear with increased bearing diameter. Curved sliding path causes a change in surface shear direction which has been shown to increase wear of polyethylene. This study confirms that increased cross-shear in the form of more angular change per linear sliding distance can increase wear of polyethylene exponentially

Objectives

Third-body wear is believed to be one trigger for adverse results with metal-on-metal (MOM) bearings. Impingement and subluxation may release metal particles from MOM replacements. We therefore challenged MOM bearings with relevant debris types of cobalt–chrome alloy (CoCr), titanium alloy (Ti6Al4V) and polymethylmethacrylate bone cement (PMMA).

Wear of polyethylene is associated with aseptic loosening of orthopaedic implants and has been observed in hip and knee prostheses and anatomical implants for the shoulder. The reversed shoulder prostheses have not been assessed as yet. We investigated the volumetric polyethylene wear of the reversed and anatomical Aequalis shoulder prostheses using a mathematical musculoskeletal model. Movement and joint stability were achieved by EMG-controlled activation of the muscles. A non-constant wear factor was considered. Simulated activities of daily living were estimated from in vivo recorded data.

After one year of use, the volumetric wear was 8.4 mm³ for the anatomical prosthesis, but 44.6 mm³ for the reversed version. For the anatomical prosthesis the predictions for contact pressure and wear were consistent with biomechanical and clinical data. The abrasive wear of the polyethylene in reversed prostheses should not be underestimated, and further analysis, both experimental and clinical, is required.