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Recently, traditional strategies for manipulating orthopedic bearing substrates have attempted to improve their wear resistance by adjusting polyethylene substrate through cross-linking and antioxidant blending. However, further research is required on the substrate, as well as the surface focused on the structure and role of articular cartilage. We therefore develop an orthopedic bearing surface comprising a nanometer-scale hydrated gel-like layer by grafting highly hydrophilic poly(2-methacryloyloxyethyl phosphorylcholine), with the aim of mimicking the lubrication mechanism of articular cartilage, and investigate its surface characteristics, bulk characteristics, and behavior under load bearing conditions upon accelerated aging. Neither the hydrophilicity nor lubricity of the gel-like surface was influenced by accelerated aging; instead, high stability was revealed, even under strong oxidation conditions. The characteristics of the hydrated gel-like surface potentiated the wear resistance of the cross-linked polyethylene liner, irrespective of accelerated aging. These results suggest that the hydrated gel-like surface enhances the longevity of cross-linked polyethylene bearings even under load-bearing conditions. Furthermore, the inflection point on the time series of wear can be a suitable indicator of the durability of the life-long protectant. In conclusion, the hydrated gel-like surface can positively increase orthopedic implant durability.

Manipulating the surface and substrate of cross-linked polyethylene (CLPE) is an essential approach for obtaining life-long orthopedic bearings. We therefore proposed a bearing material comprised of an antioxidative substrate generated by vitamin E blending (HD-CLPE[VE]) with a poly(2-methacryloyloxyethyl phosphorylcholine) (PMPC)-grafted surface, and investigated its wear resistance and oxidative stability under accelerated aging and load bearing conditions. Neither the hydration nor friction kinetics of the molecular network structure of the PMPC-grafted surface or the HD-CLPE(VE) substrate were influenced by accelerated aging but rather exhibited high stability even under high oxidation conditions. The characteristics of the PMPC-grafted surface improved the wear and impact fatigue resistance of the HD-CLPE(VE) liner regardless of accelerated aging. Notably, the PMPC-grafted surface was found to affect the potential oxidative stability at the rim part of the acetabular liner. PMPC chains serve several important functions on the surface regardless of load bearing, such as high lubricity or low lipophilicity attributed to phosphorylcholine groups and/or surrounding water-fluid film, and suppression of lipid diffusion attributed to methacrylate main chains on the surface. Together, these results provide preliminary evidence that the PMPC graft layer and vitamin E-blended substrate might positively affect the extent of orthopedic implant durability., (Copyright © 2017 Elsevier Ltd. All rights reserved.)

One of the important factors in determining the success of joint replacement is the wear performance of polyethylene. Although highly crosslinked polyethylene (CLPE) is presently used, it is still not adequate. We have developed a surface modification technology using poly(2-methacryloyloxyethyl phosphorylcholine) (PMPC) in an attempt to improve wear performance. In this study, we evaluated the wear and creep deformation resistances of 3-mm and 6-mm thick PMPC-grafted CLPE disks, set on a metal back-plate, with and without a sham screw hole. The gravimetric wear and volumetric change of the disks were examined using a multidirectional pin-on-disk tester. PMPC grafting decreased the gravimetric wear of CLPE regardless of the presence of a screw hole, and did not affect the volumetric change. The volumetric change in the bearing and backside surfaces of the 3-mm thick disk with a screw hole was much larger than that of those without a screw hole or those of the 6-mm thick disk, which was caused by creep deformation. PMPC grafting on the bearing surface can be a material engineering approach to reduce the wear without changing the creep deformation resistance, and is a promising surface modification technology that can be used to increase the longevity of various artificial joints. © 2017 Wiley Periodicals, Inc. J Biomed Mater Res Part B: Appl Biomater, 106B: 610-618, 2018., (© 2017 Wiley Periodicals, Inc.)

The surface and substrate of a cross-linked polyethylene (CLPE) liner are designed to achieve resistance against oxidative degradation in the construction of hip joint replacements. In this study, we aimed to evaluate the oxidative degradation caused by lipid absorption of a highly hydrophilic nanometer-scaled thickness layer prepared by grafting a poly(2-methacryloyloxyethyl phosphorylcholine) (PMPC) layer and a high-dose gamma-ray irradiated CLPE with vitamin E blending (HD-CLPE[VE]). The HD-CLPE(VE) and PMPC-grafted HD-CLPE(VE) exhibited extremely high oxidation resistance regardless of lipid absorption, even though residual-free radical levels were detectable. The water wettability of the PMPC-grafted CLPE and PMPC-grafted HD-CLPE(VE) surfaces was considerably greater than that of untreated surfaces. The hydrated PMPC-grafted layer also exhibited extremely low solubility for squalene. Lipids such as squalene and cholesterol esters diminished the oxidation resistance of CLPE despite the vitamin E improvement. Notably, the PMPC-grafted surface was resistant to lipid absorption and diffusion as well as subsequent lipid-related oxidative degradation, likely because of the presence of the hydrated PMPC-grafted layer. Together, these results provide preliminary evidence that the resistance against lipid absorption and diffusion of a hydrated PMPC-grafted layer might positively affect the extent of resistance to the in vivo oxidation of orthopedic implants., (Copyright © 2016 Elsevier Ltd. All rights reserved.)

Sterilization using high-energy irradiation is an important aspect of implementing an ultra-high molecular weight polyethylene acetabular liner in total hip arthroplasty (THA). In this study, we evaluate the effects of extra irradiations such as gamma-ray or plasma irradiation during sterilization of the poly(2-methacryloyloxyethyl phosphorylcholine [MPC]) (PMPC) surface and cross-linked polyethylene (CLPE) substrate of a PMPC-grafted CLPE acetabular liner. The PMPC-grafted surface yielded high wettability and low friction properties regardless of the extra irradiations as compared with untreated CLPE. During a hip simulator test, wear resistance of the PMPC-grafted CLPE liner was maintained after extra irradiation, which is due to the high wettability characteristics of the PMPC surface. In particular, the PMPC-grafted CLPE liner treated with plasma irradiation showed greater wettability and wear resistance than that with gamma-ray irradiation. However, we could not clearly observe the changes in chemical properties and morphology of the PMPC surface after both extra irradiations. The physical and mechanical properties attributed to CLPE substrate performance were also unchanged. In contrast, PMPC-grafted CLPE treated with plasma irradiation showed improved oxidation resistance as compared to that treated with gamma-ray irradiation after accelerated aging. Thus, we conclude that PMPC-grafted CLPE with plasma irradiation has promise as a lifelong solution for bearing in THA., (© 2015 Wiley Periodicals, Inc.)

In the construction of artificial hip joint replacements, the surface and substrate of a cross-linked polyethylene (CLPE) liner are designed to achieve high wear resistance and prevent infection by bacteria. In this study, we fabricated a highly hydrophilic and antibiofouling poly(2-methacryloyloxyethyl phosphorylcholine [MPC]) (PMPC)-graft layer on the vitamin E-blended CLPE (HD-CLPE(VE)) surface. The 100-nm-thick, smooth, and electrically neutral PMPC layer was successfully fabricated on the HD-CLPE(VE) surface using photoinduced graft polymerization. The PMPC-grafted HD-CLPE(VE) was found to prevent bacterial adherence and biofilm formation on the surface because of the formation of a highly hydrophilic polyzwitterionic layer on the surface of HD-CLPE(VE), which can serve as an extremely efficient antibiofouling layer. The number of bacterial adhered on the PMPC-grafted HD-CLPE(VE) surface was reduced by 100-fold or more by PMPC grafting, regardless of the biofilm-production characteristics of the strains. In contrast, vitamin E blending did not affect bacterial adhesion. Moreover, the number of planktonic bacteria did not differ significantly, regardless of PMPC grafting and vitamin E blending. In conclusion, the PMPC-grafted HD-CLPE(VE) provided bacteriostatic effects associated with smooth, highly hydrophilic surfaces with a neutral electrostatic charge owing to the zwitterionic structure of the MPC unit. Thus, this modification may prove useful for the production of artificial hip joint replacement materials., Statement of Significance: Our preliminary in vitro findings suggest that improved bacteriostatic performance of the HD-CLPE(VE) surface in orthopedic implants is possible via PMPC grafting. The results also indicate that surface modifications affect the anti-infection properties of the orthopedic implants and demonstrate that the application of a PMPC-grafted HD-CLPE(VE) surface may be a promising approach to extend the longevity and clinical outcomes of total hip arthroplasty. Further research is needed to evaluate the resistance to infection of PMPC-grafted HD-CLPE(VE) in terms of the varieties of biofilm formation tests including fluid flow conditions and animal experiments, which may offer useful clues to the possible performance of these materials in vivo., (Copyright © 2015 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.)

Surface modification by grafting of biocompatible phospholipid polymer onto the surface of artificial joint material has been proposed to reduce the risk of aseptic loosening and improve the durability. Poly(2-methacryloyloxyethyl phosphorylcholine) (PMPC)-grafted cross-linked polyethylene (CLPE) has shown promising results for reducing wear of CLPE. The main lubrication mechanism for the PMPC layer is considered to be the hydration lubrication. In this study, the lubrication properties of PMPC-grafted CLPE were evaluated in reciprocating friction test with rehydration process by unloading in various lubricants. The start-up friction of PMPC-grafted CLPE was reduced, and the damage of PMPC layer was suppressed by rehydration in water or hyaluronic acid solutions. In contrast, the start-up friction of PMPC-grafted CLPE increased in fetal bovine serum solution, and the damage for PMPC layer was quite noticeable. Interestingly, the start-up friction of PMPC-grafted CLPE was reduced in fetal bovine serum solution containing hyaluronic acid, and the damage of the PMPC layer was suppressed. These results indicate that the rehydration by unloading and hyaluronic acid are elemental in maximizing the lubrication effect of hydrated PMPC layer., (© IMechE 2015.)

The use of larger femoral heads to prevent the dislocation of artificial hip joints has recently become more common. However, concerns about the subsequent use of thinner polyethylene liners and their effects on wear rate have arisen. Previously, we prepared and evaluated the biological and mechanical effects of a novel highly cross-linked polyethylene (CLPE) liner with a nanometer-scaled graft layer of poly(2-methacryloyloxyethyl phosphorylcholine) (PMPC). Our findings showed that the PMPC-grafted particles were biologically inert and caused no subsequent bone resorptive responses and that the PMPC-grafting markedly decreased wear in a hip joint simulator. However, the metal or ceramic femoral heads used in this previous study had a diameter of 26 mm. Here, we investigated the wear-resistance of the PMPC-grafted CLPE liner with a 40-mm femoral head during 10 × 10(6) cycles of loading in the hip joint simulator. The results provide preliminary evidence that the grafting markedly decreased gravimetric wear rate and the volume of wear particles, even when coupled with larger femoral heads. Thus, we believe the PMPC-grafting will prolong artificial hip joint longevity both by preventing aseptic loosening and by improving the stability of articular surface., (© 2015 Orthopaedic Research Society. Published by Wiley Periodicals, Inc.)

The wear and creep deformation resistances of polymeric orthopedic bearing materials are both important for extending their longevity. In this study, we evaluated the wear and creep deformation resistances, including backside damage, of different polyethylene (PE) materials, namely, conventional PE, cross-linked PE (CLPE), and poly(2-methacryloyloxyethyl phosphorylcholine)- (PMPC-) grafted CLPE, through wear tests and finite element analysis. The gravimetric and volumetric degrees of wear of disks (3 or 6 mm in thickness) of these materials against a cobalt-chromium-molybdenum alloy pin were examined using a multidirectional pin-on-disk tester. Cross-linking and PMPC grafting decreased the gravimetric wear of the PE disks significantly. The volumetric wear at the bearing surface and the volumetric penetration in the backside of the 3-mm thick PE disk were higher than those of the 6-mm thick PE disk, regardless of the bearing material. The geometrical changes induced in the PE disks consisted of creep, because the calculated internal von Mises stress at the bearing side of all disks and that at the backside of the 3-mm thick disks exceeded their actual yield strengths. A highly hydrated bearing surface layer, formed by PMPC grafting, and a cross-linking-strengthened substrate of adequate thickness are essential for increasing the wear and creep deformation resistances.

Photoinduced grafting of 2-methacryloyloxyethyl phosphorylcholine (MPC) onto cross-linked polyethylene (CLPE) was investigated for its ability to reduce the wear of orthopedic bearings. We investigated the effect of UV-irradiation intensity on the extent of poly(MPC) (PMPC) grafting, and found that it increased with increasing intensity up to 7.5 mW/cm(2), and the remained fairly constant. It was found to be extremely important to carefully control the UV intensity, as at higher values, a PMPC gel formed via homopolymerization of the MPC, resulting in the formation of cracks at the interface of the PMPC layer and the CLPE substrate. When the CLPE was exposed to UV-irradiation during the graft polymerization process, some of its physical and mechanical properties were slightly changed due to cross-linking and scission effects in the surface region; however, the results of all of the tests exceed the lower limits of the ASTM standards. Modification of the CLPE surface with the hydrophilic PMPC layer increased lubrication to levels that match articular cartilage. The highly hydrated thin PMPC films mimicked the native cartilage extracellular matrix that covers synovial joint surface, acting as an extremely efficient lubricant, and providing high-wear resistance., (© 2013 Wiley Periodicals, Inc.)

Despite improvements in the techniques, materials, and fixation of total hip arthroplasty, periprosthetic osteolysis, a complication that arises from this clinical procedure and causes aseptic loosening, is considered to be a major clinical problem associated with total hip arthroplasty. With the objective of reducing the production of wear particles and eliminating periprosthetic osteolysis, we prepared a novel hip polyethylene (PE) liner whose surface graft was made of a biocompatible phospholipid polymer-poly(2-methacryloyloxyethyl phosphorylcholine (MPC)). This study investigated the wear resistance of the poly(MPC)-grafted cross-linked PE (CLPE; MPC-CLPE) liner during 15×10(6) cycles of loading in a hip joint simulator. The gravimetric analysis showed that the wear of the acetabular liner was dramatically suppressed in the MPC-CLPE liner, as compared to that in the non-treated CLPE liner. Analyses of the MPC-CLPE liner surface revealed that it suffered from no or very little wear even after the simulator test, whereas the CLPE liners suffered from substantial wears. The scanning electron microscope (SEM) analysis of the wear particles isolated from the lubricants showed that poly(MPC) grafting dramatically decreased the total number, area, and volume of the wear particles. However, there was no significant difference in the particle size distributions, and, in particular, from the SEM image, it was observed that particles with diameters less than 0.50μm were present in the range of the highest frequency. In addition, there were no significant differences in the particle size descriptors and particle shape descriptors. The results obtained in this study show that poly(MPC) grafting markedly reduces the production of wear particles from CLPE liners, without affecting the size of the particles. These results suggest that poly(MPC) grafting is a promising technique for increasing the longevity of artificial hip joints., (Copyright © 2013 Elsevier Ltd. All rights reserved.)

To prevent periprosthetic osteolysis and subsequent aseptic loosening of artificial hip joints, we recently developed a novel acetabular highly cross-linked polyethylene (CLPE) liner with graft polymerization of 2-methacryloyloxyethyl phosphorylcholine (MPC) on its surface. We investigated the wear resistance of the poly(MPC) (PMPC)-grafted CLPE liner during 20 million cycles in a hip joint simulator. We extended the simulator test of one liner to 70 million cycles to investigate the long-term durability of the grafting. Gravimetric, surface, and wear particle analyses revealed that PMPC grafting onto the CLPE liner surface markedly decreased the production of wear particles and showed that the effect of PMPC grafting was maintained through 70 million cycles. We believe that PMPC grafting can significantly improve the wear resistance of artificial hip joints., (© 2013 Orthopaedic Research Society. Published by Wiley Periodicals, Inc.)

Natural joints rely on fluid thin-film lubrication by the hydrated polyelectrolyte layer of cartilage. However, current artificial joints with polyethylene (PE) surfaces have considerably less efficient lubrication and thus much greater wear, leading to osteolysis and aseptic loosening. This is considered a common factor limiting prosthetic longevity in total hip arthroplasty (THA). However, such wear could be mitigated by surface modification to mimic the role of cartilage. Here we report the development of nanometer-scale hydrophilic layers with varying charge (nonionic, cationic, anionic, or zwitterionic) on cross-linked PE (CLPE) surfaces, which could fully mimic the hydrophilicity and lubricity of the natural joint surface. We present evidence to support two lubrication mechanisms: the primary mechanism is due to the high level of hydration in the grafted layer, where water molecules act as very efficient lubricants; and the secondary mechanism is repulsion of protein molecules and positively charged inorganic ions by the grafted polyelectrolyte layer. Thus, such nanometer-scaled hydrophilic polymers or polyelectrolyte layers on the CLPE surface of acetabular cup bearings could confer high durability to THA prosthetics., (Copyright © 2012 Elsevier Ltd. All rights reserved.)

Surface modification is important for the improvement in medical device materials. 2-Methacryloyloxyethyl phosphorylcholine (MPC) polymers have attracted considerable attention as surface modifiable polymers for several medical devices. In this study, we hypothesize that the structure of the surface modification layers might affect the long-term stability, hydration kinetics, wear resistance, and so forth, of medical devices such as artificial joints, and the poly(MPC) (PMPC) grafted surface might assure the long-term performance of such devices. Therefore, we investigate the surface properties of various surface modifications by using dip coatings of MPC-co-n-butyl methacrylate (PMB30) and MPC-co-3-methacryloxypropyl trimethoxysilane (PMSi90) polymers, or photoinduced radical grafting of PMPC and also the effects of the surface properties on the durability of cross-linked polyethylene (CLPE) for artificial joints. The PMPC-grafted CLPE has an extremely low and stable coefficient of dynamic friction and volumetric wear as compared to the untreated CLPE, PMB30-coated CLPE, and PMSi90-coated CLPE. It is concluded that the photoinduced radical graft polymerization of MPC is the best method to retain the benefits of the MPC polymer used in artificial joints under variable and multidirectional loads for long periods with strong bonding between the MPC polymer and the CLPE surface, and also to retain the high mobility of the MPC polymer.

Osteolysis caused by wear particles from polyethylene in the artificial hip joints is a serious issue. We have used photo-induced radical graft polymerization to graft 2-methacryloyloxyethyl phosphorylcholine (MPC) polymer onto the surface of cross-linked polyethylene (CLPE-g-MPC) in order to reduce friction and wear at the bearing surface of the joint. The physical and mechanical properties of CLPE and CLPE-g-MPC were not significantly different, expect that the friction coefficient of untreated CLPE cups was 0.0075, compared with 0.0009 for CLPE-g-MPC cup, an 88% reduction. After 3.0 x 10(6) cycles in the hip joint simulator test, we could not observe any wear of CLPE-g-MPC cups. We concluded that the advantage of photo-induced radical graft polymerization technique was that the grafted MPC polymer gave a high lubricity only on the surface and has no effect on the bulk properties of the CLPE substrate.

Aseptic loosening induced by wear particles from the polyethylene liner is likely the most common cause of long-term total hip arthroplasty failure. We developed a novel hip polyethylene liner with the surface graft of a biocompatible phospholipid polymer, 2-methacryloyloxyethyl phosphorylcholine (MPC), and previously reported the grafting decreased the short-term production of wear particles and the subsequent bone resorptive responses. For clinical application, we investigated the stability of the 2-methacryloyloxyethyl phosphorylcholine grafting during sterilization and the wear resistance of the sterilized liner during longer loading comparable to clinical usage. Radiographic spectroscopy confirmed the stability of the 2-methacryloyloxyethyl phosphorylcholine polymer on the liner surface after the gamma irradiation. We used a hip wear simulator up to 1 x 10(7) cycles to test sterilized cross-linked polyethylene liners with and without 2-methacryloyloxyethyl phosphorylcholine grafting. The 2-methacryloyloxyethyl phosphorylcholine grafting markedly decreased the friction, the production of wear particles, and the wear of the liner surface. These data suggest a marked improvement in the wear resistance of the polyethylene liner by the 2-methacryloyloxyethyl phosphorylcholine grafting for clinically relevant periods after sterilization, indicating 2-methacryloyloxyethyl phosphorylcholine grafting is a promising technology for extending longevity of artificial hips.

Summary: Objective: Aseptic loosening of artificial knee joints induced by wear particles from a tibial polyethylene (PE) insert is a serious problem limiting their longevity. This study investigated the effects of grafting with our original biocompatible phospholipid polymer 2-methacryloyloxyethyl phosphorylcholine (MPC) on the insert surface. Methods: The hydrophilicity of the PE surface was determined by the contact angle of a water droplet, and the friction torque was measured against a cobalt–chromium alloy component. The wear amount was compared among PE inserts with or without cross-linking and MPC grafting during 5×106 cycles of loading in a knee joint simulator. The surfaces of the insert and the wear particles in the lubricant were subjected to electron and laser microscopic analyses. The mechanical properties of the inserts were evaluated by the small punch test. Results: The MPC grafting increased hydrophilicity and decreased friction torque. In the simulator experiment, the wear of the tibial insert was significantly suppressed in the cross-linked PE (CLPE) insert, and even more dramatically decreased in the MPC-grafted CLPE insert, as compared to that in the non-cross-linked PE insert. Surface analyses confirmed the wear resistance by the cross-linking, and further by the MPC grafting. The particle size distribution was not affected by cross-linking or MPC grafting. The mechanical properties of the insert material remained unchanged during the loading regardless of the cross-linking or grafting. Conclusion: Surface grafting with MPC polymer furnished the PE insert with wear resistance in an artificial knee joint through increased hydrophilicity and decreased friction torque. [Copyright &y& Elsevier]