Original Article
Influences of dehydration and rehydration on the lubrication properties of phospholipid polymergrafted cross-linked polyethylene
Proc IMechE Part H: J Engineering in Medicine 2015, Vol. 229(7) 506–514 Ó IMechE 2015 Reprints and permissions: sagepub.co.uk/journalsPermissions.nav DOI: 10.1177/0954411915588969 pih.sagepub.com
Seido Yarimitsu1,2, Toru Moro3,4, Masayuki Kyomoto3,5,6, Kenichi Watanabe3,6, Sakae Tanaka4, Kazuhiko Ishihara5 and Teruo Murakami1
Abstract 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 PMPCgrafted 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.
Keywords Artificial joint, MPC polymer, hydration lubrication, wear, friction
Date received: 24 September 2014; accepted: 5 May 2015
Introduction The number of total joint replacements is increasing every year.1 This type of surgery can relieve serious pain and improve quality of life. The most popular common bearing surface for artificial joints is a combination of an ultra-high molecular weight polyethylene (UHMWPE) and a cobalt–chromium–molybdenum (Co–Cr–Mo) alloy. The materials (i.e. cross-linked polyethylene (CLPE)) of artificial joints have been continually improved with the aim of reducing the UHMWPE wear particles. However, the three big complications limiting survivorship and clinical success,2–5 that is, dislocation, infection, and aseptic loosening, are not still solved completely. Hence, numerous attempts have been made to improve the wear resistance of the UHMWPE substrate. The cross-linking by gamma-ray irradiation is one of the effective treatments for reduction of wear of UHMWPE.6–8 Furthermore, vitamin E blending or diffusion is effective in preventing the
oxidative degradation and delamination wear of UHMWPE.9,10 However, those improvements to the UHMWPE substrate have not yet completely solved the wear-related problems. To prevent periprosthetic osteolysis and subsequent aseptic loosening, Moro et al.11 have studied the 1
Research Center for Advanced Biomechanics, Kyushu University, Fukuoka, Japan 2 Faculty of System Design, Tokyo Metropolitan University, Tokyo, Japan 3 Division of Science for Joint Reconstruction, Graduate School of Medicine, The University of Tokyo, Tokyo, Japan 4 Sensory & Motor System Medicine, Faculty of Medicine, The University of Tokyo, Tokyo, Japan 5 Department of Materials Engineering, School of Engineering, The University of Tokyo, Tokyo, Japan 6 Research Department, KYOCERA Medical Corporation, Osaka, Japan Corresponding author: Seido Yarimitsu, Faculty of System Design, Tokyo Metropolitan University, 6-6 Asahigaoka, Hino, Tokyo 191-0065, Japan. Email: [email protected]
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lubrication mechanism of natural synovial joint and the surface structure of natural articular cartilage. Natural articular cartilage has a hydration gel layer and phospholipid layer on its surface,12 and these surface layers contribute to the excellent lubrication property of natural synovial joint. Therefore, they have developed a new artificial joint system with a 100-nm-thick surface hydration layer formed by poly(2-methacryloyloxyethyl phosphorylcholine (MPC)) (PMPC)13 grafted onto the surface of CLPE (PMPC-grafted CLPE),11 mimicking the surface layer of natural articular cartilage.12 In previous hip and knee joint simulator studies, PMPCgrafted CLPE showed extremely low wear.14–17 Especially, the effect of PMPC grafting was maintained up to 70 million cycles in the hip simulator test.18 In addition, the PMPC-grafted particles were biologically inert and did not cause subsequent bone-resorptive responses.11 In the follow-up study of clinical trials of PMPC-grafted CLPE, no complication derived from PMPC-grafted CLPE occurred and the linear wear rate of PMPC-grafted CLPE was one-ordered smaller than other conventional CLPEs.19,20 Therefore, PMPCgrafted CLPE should contribute dramatically toward the improved longevity of artificial hip joint. The lubrication using a hydrated gel layer formed by hydrophilic polymer on the rubbing surface is important in maintaining low friction.21,22 In aqueous solution, the hydrophilic macromolecules on the bearing surfaces attract water molecules and form a water-rich hydrated gel layer. If a load is applied to the surface, it is mostly supported by the fluid pressure in the hydrated gel layer. In the case of sliding motion, water molecules are sheared with low stress, and low friction is maintained until the water is discharged by loading from the hydrated gel layer.23 This mechanism is known as hydration lubrication. Excellent lubricity and durability of the PMPC-grafted CLPE are exhibited, owing to this hydration lubrication. Therefore, it is important to elucidate the details pertaining to hydration lubrication of the PMPC layer in the bearing interface for further improvement in the longevity of artificial joints. In this study, we therefore investigated the effects of dehydration and rehydration by loading–unloading procedure on lubrication properties of PMPC-grafted CLPE using various lubricants. An in situ visualization technique was used in simplified reciprocating friction tests to track changes in the surface of PMPC-grafted CLPE. Throughout our studies, we questioned whether the hydration kinetics of the hydrated PMPC layer would affect the friction kinetics of the PMPC-grafted CLPE surface.
Germany) bar stock was irradiated with a 50-kGy dose of gamma-rays in a N2 gas, and annealed at 120 °C for 7.5 h in N2 gas in order to facilitate cross-linking. The resulting CLPE pin specimens were then machined from this bar stock after cooling. The pin geometry was a flat-ended, conical/cylindrical shape (u6 3 9 mm) with a tip diameter of u2 mm. The rubbing surfaces of CLPE specimens were polished and the arithmetical mean roughness (Ra) was approximately 0.4 mm.
Materials and methods CLPE pin specimen
In situ visualization on PMPC layers in friction tests with the rehydration procedure
A compression-molded UHMWPE (GUR1020 resin; Quadrant PHS Deutschland GmbH, Vreden,
A schematic illustration of the reciprocating friction tester used in this study is shown in Figure 1(a). The
PMPC-grafted CLPE pin specimen The CLPE pin specimens were immersed in acetone (Wako Pure Chemical Industries, Ltd, Osaka, Japan) containing 10 mg/mL benzophenone (Wako Pure Chemical Industries, Ltd) for 30 s, and then dried in the dark at room temperature in order to remove the acetone. Industrially synthesized MPC was purchased from NOF Corp. (Tokyo, Japan).13 The MPC was dissolved in degassed pure water to a concentration of 0.5 mol/L. Subsequently, the benzophenone-coated CLPE specimens were immersed in the MPC aqueous solutions. Photo-induced graft polymerization was carried out on the CLPE surface using ultra-violet (UV) irradiation (UVL-400HA ultra-high pressure mercury lamp; Riko-Kagaku Sangyo Co., Ltd, Funabashi, Japan) with an intensity of 5.0 mW/cm2 at 60 °C for 90 min; a filter (model D-35; Toshiba Corp., Tokyo, Japan) was used to restrict the passage of UV light to wavelengths of 350 6 50 nm.15 After the polymerization, the PMPC-grafted CLPE pin specimens were removed, washed with pure water and ethanol, and dried at room temperature. All obtained specimens were then sterilized by 25 kGy gamma-rays in a N2 gas.
Single-crystal sapphire A single-crystal sapphire plate (KYOCERA Corporation, Kyoto, Japan) was used as a counterface against the untreated and PMPC-grafted CLPE pins owing to observations of the frictional surface through the transparent sapphire plate during the friction test. The surface of the sapphire plate was mirror-finished, and its Ra was approximately 0.05 mm.
Fluorescent labeling of PMPC For in situ visualization of the PMPC layer, the PMPCgrafted layer was fluorescently labeled with rhodamine 6G (Wako Pure Chemical Industries, Ltd). The PMPCgrafted CLPE pin specimens were immersed in an aqueous solution of 200 mass ppm rhodamine 6 G for 30 s and then removed. These were washed twice in distilled water for 30 s and then dried.
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Proc IMechE Part H: J Engineering in Medicine 229(7) grafted CLPE pin specimens were soaked in each lubricant for 10 min for hydration of PMPC layer. The reciprocating test was initiated immediately after applying a load, and the reciprocating motion continued for five cycles (5 s). After the reciprocating cycles, the upper pin specimen was separated from the counterface and was kept with unloading for 60 s in a lubricant. Then, the friction test was restarted, and these procedures were repeated until a total sliding distance of 2.0 m was reached. At sliding distances of 0.05, 0.25, 0.50, 1.0, and 2.0 m, the PMPC layer on the sliding surface was imaged using fluorescence microscopy.
Statistical analyses The mean values for the friction coefficients of the four groups (untreated CLPE and PMPC-grafted CLPE with/without rehydration) were compared by a onefactor analysis of variance (ANOVA), and the significant differences (p \ 0.05) of the all comparable properties were determined by post hoc testing using Tukey’s method. All statistical analyses were performed using a KaleidaGraph (Synergy Software, Philadelphia, PA, USA).
Results In pure water
Figure 1. Reciprocating friction test for in situ visualization of a sliding surface: (a) details of the reciprocating friction tester and (b) schematic illustration of a reciprocating friction test with a rehydration procedure.
reciprocating friction tester was constructed on the stage of an inverted fluorescence microscope (IX-71, Olympus Corporation, Tokyo, Japan). Sliding pairs of the untreated or PMPC-grafted CLPE pin specimen and the sapphire plate were evaluated by using the reciprocating friction tester, and the sliding surface of the PMPC-grafted CLPE pin specimens was observed through the transparent sapphire plate. The reciprocating friction tests were performed at room temperature with a vertical load of 7.84 N (mean contact pressure was 2.50 MPa), a sliding speed of 10 mm/s, and a reciprocating stroke of 10 mm. The total sliding distance was 2.0 m. Four lubricants were used in this study: pure water, 0.5 mass% hyaluronic acid (HA; Mw = 9.2 3 105) aqueous solution, 30 vol.% fetal bovine serum (FBS; Invitrogen Corporation, Carlsbad, CA, USA) solution, and a mixture of 30 vol.% FBS and 0.5 mass% HA. A schematic illustration of the reciprocating friction test with the rehydration procedure is shown in Figure 1(b). Before the reciprocating test, the PMPC-
The friction coefficients of each of the untreated and PMPC-grafted CLPE in pure water are shown in Figure 2. The start-up friction of PMPC-grafted CLPE was significantly lower than that of CLPE, regardless of rehydration (Figure 2(a) and (b)). With the rehydration procedures, the PMPC-grafted CLPE maintained a lower friction during the 2.0-m test (Figure 2(a) and (c)). The recovery and gradual increase in friction coefficient for each rehydration procedure was observed both in cases of untreated and PMPC-grafted CLPE. In contrast, the friction coefficient of PMPC-grafted CLPE without rehydration was nearly identical with that of CLPE. The fluorescence microscopy images of the PMPC layer are shown in Figure 3. With rehydration procedure, damage of the PMPC layer progressed partially in the convex region of the machine marks (arc-like patterns) on the PMPC-grafted CLPE surface, with increasing sliding distance. In contrast, the PMPC layer on the convex region of the machine marks was severely damaged without rehydration procedures.
In HA solution When HA solution was used as lubricant, the start-up friction of PMPC-grafted CLPE was lower than that of CLPE, regardless of rehydration (Figure 4(a) and (b)). However, there was no significant difference in the friction coefficient at steady state between untreated and PMPC-grafted CLPE (Figure 4(c)). Friction of the PMPC-grafted CLPE recovered slightly with each of
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Figure 2. Friction coefficient in pure water: (a) transient of friction coefficient, (b) friction coefficient at the 1st cycle in the reciprocating friction test, and (c) friction coefficient at a sliding distance of 2.0 m. Values: mean 6 standard deviation (n = 5). *p \ 0.05 (Tukey’s HSD test).
Figure 3. Fluorescence microscopy images of the PMPC layer lubricated in pure water (numbers in the subcaption indicate sliding distance, bars: 50 mm, arrows: sliding direction): (a) 0.05 m, (b) 0.25 m, (c) 0.50 m, (d) 1.00 m, (e) 2.00 m, and (f) 2.00 m.
the rehydration procedures, but there was no significant difference in the friction coefficient of the PMPCgrafted CLPE in cases with and without rehydration. The fluorescence microscopy images of the PMPC layer lubricated in the HA solution are shown in Figure 5. The damage of the PMPC layer in HA solution was slight, despite the lack of rehydration procedure (Figure 5(f)), and the suppression of damage of the PMPC layer was confirmed when rehydration procedures were provided, even after 2.0 m sliding tests.
In FBS solution Figure 6 shows the friction coefficient of the untreated and PMPC-grafted CLPE in 30 vol.% FBS solution. The start-up friction of PMPC-grafted CLPE was lower than that of CLPE (Figure 6(a) and (b)). As shown in Figure 6(a) and (c), the PMPC-grafted CLPE with
rehydration maintained a lower friction coefficient than that of untreated CLPE with/without rehydration and PMPC-grafted CLPE without rehydration. The friction coefficient of PMPC-grafted CLPE without rehydration gradually increased and approached asymptotically to that of CLPE. The damage of the PMPC layer was limited to the convex region of the machine marks on the surface of PMPC-grafted CLPE when rehydration procedures were performed (Figure 7). In contrast, in the case without rehydration, damage of the PMPC layer ranged from the convex to concave regions of the machine marks on the PMPC-grafted CLPE surface.
In a mixture of FBS and HA solution Figure 8 shows the friction coefficient of the untreated and PMPC-grafted CLPE in a mixture of 30 vol.% FBS and 0.5 mass% HA. Both the start-up and steady-
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Figure 4. Friction coefficient in HA 0.5 mass% solution: (a) transient of friction coefficient, (b) friction coefficient at the 1st cycle in the reciprocating friction test, and (c) friction coefficient at a sliding distance of 2.0 m. Values: mean 6 standard deviation (n = 5). *p \ 0.05 (Tukey’s HSD test).
Figure 5. Fluorescence microscopy images of the PMPC layer lubricated in HA 0.5 mass% solution (numbers in the subcaption indicate sliding distance, bars: 50 mm, arrows: sliding direction): (a) 0.05 m, (b) 0.25 m, (c) 0.50 m, (d) 1.00 m, (e) 2.00 m, and (f) 2.00 m.
state friction of the PMPC-grafted CLPE was lower than that of CLPE, regardless of rehydration. The friction coefficient of untreated and PMPC-grafted CLPE rose and then gradually decreased during each rehydration. There was no difference in the friction coefficient of the PMPC-grafted CLPE for cases with and without hydration. Damages of the PMPC layer on the convex region of the machine marks on the PMPC-grafted CLPE surface were suppressed with rehydration (Figure 9).
Discussion In this study, we investigated the effects of dehydration and rehydration by loading–unloading procedure on the lubrication properties of PMPC-grafted CLPE in
various lubricants by using an in situ visualization technique with simplified reciprocating friction. At the beginning of this study, we questioned whether the hydration kinetics of a hydrated PMPC layer would affect the friction kinetics of a PMPC-grafted CLPE surface. The friction properties of PMPC-grafted CLPE, in terms of the rehydration effects of the PMPC layer, will be discussed hereafter. The hydrated PMPC layer clearly affected the friction response; the friction coefficients for the start-up motion of the PMPC-grafted CLPE surfaces were significantly lower than those of the untreated CLPE surface with every lubricant tested. This behavior is attributed to the significant increase in hydrophilicity that is evident from the reduction in the static watercontact angles of the PMPC-grafted surfaces.11,14
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Figure 6. Friction coefficient in FBS 30 vol.% solution: (a) transient of friction coefficient, (b) friction coefficient at the 1st cycle in the reciprocating friction test, and (c) friction coefficient at a sliding distance of 2.0 m. Values: mean 6 standard deviation (n = 5). *p \ 0.05 (Tukey’s HSD test).
Figure 7. Fluorescence microscopy images of the PMPC layer lubricated in FBS 30 vol.% solution (numbers in the subcaption indicate sliding distance, bars: 50 mm, arrows: sliding direction): (a) 0.05 m, (b) 0.25 m, (c) 0.50 m, (d) 1.00 m, (e) 2.00 m, and (f) 2.00 m.
The reduction of the friction coefficient of PMPCgrafted CLPE by rehydration in the steady state was observed only in pure water and in HA solution. These results may imply that the lubrication of PMPC-grafted CLPE was dominated by the hydration–lubrication mechanism. However, the PMPC layer gradually lost its effect under continuous loading. When the load is applied continuously or for long periods, water molecules in the PMPC layer are squeezed out. Thus, the effects of hydration lubrication may be diminished, and dehydration likely leads to an increase in friction as well as damage of the PMPC layer as a result. Therefore, ensuring rehydration by unloading or load variation is essential in order to establish and maintain the effect of hydration lubrication of the PMPC layer. The friction coefficient in HA solution was higher than
that in pure water, but the damage of PMPC layer was suppressed by addition of HA. It was thought that the increase in friction by HA additives was attributed to the viscous resistance of HA solution and/or adsorbed HA layer. However, HA would contribute to the separation of frictional mating surfaces by enhancement of fluid film formation or formation of adsorbed film. The untreated CLPE, lubricated in water or HA solution, also showed significant reduction in its friction coefficient after unloading despite having no hydration layer on its surface. This is because the conformity of a bearing interface is improved by creep deformation of the CLPE substrate. At the initial state including start-up of the friction test, the friction coefficients of both untreated and PMPC-grafted CLPE decreased gradually with an increase in sliding distance.
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Figure 8. Friction coefficient in a mixture of FBS 30 vol.% and HA 0.5 mass% solution: (a) transient of friction coefficient, (b) friction coefficient at the 1st cycle in the reciprocating friction test, and (c) friction coefficient at a sliding distance of 2.0 m. Values: mean 6 standard deviation (n = 5). *p \ 0.05 (Tukey’s HSD test).
Figure 9. Fluorescence microscopy images of the PMPC layer lubricated in a mixture of FBS 30 vol.% and HA 0.5 mass% solution (numbers in the subcaption indicate sliding distance, bars: 50 mm, arrows: sliding direction): (a) 0.05 m, (b) 0.25 m, (c) 0.50 m, (d) 1.00 m, (e) 2.00 m, and (f) 2.00 m.
In the hydration–lubrication model, it is well known that the friction coefficient gradually increases after applying load and reaches plateau.22 Hence, it may be the case that the running-in effect of the surfaces of PMPC-grafted CLPE and CLPE overshadows the effect of hydration lubrication during the initial-state friction test. In FBS solution, an increasing of friction coefficient after rehydration was clearly observed. This was the characteristic phenomenon for FBS solution. It was expected that an adsorbed film would be formed on the surface of untreated and PMPC-grafted CLPE since FBS contains several proteins (e.g. albumin and globulin) and lipids. After the boundary films were formed through rubbing and the applied load was removed,
further proteins and lipids would adsorb on the rubbed surfaces. Immediately after reloading, the excessively adsorbed molecules may interact with high friction, but during continuous rubbing, the boundary films appear to be reoriented by frictional force. It may be the case that a decrease in friction coefficient after restarting frictional motion is due to this reorientation of adsorbed film by rubbing. In the FBS solution, the damage of PMPC layer was accelerated compared with that of water or HA lubricants. It is well established that a hydrated PMPC layer prevents protein adsorption because of zwitterionic polyelectrolyte.24 Moreover, the zeta potential of the PMPC-grafted CLPE surface was close to zero because the ionic group in the MPC unit forms an inner salt and the
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electrostatic effects are diminished. Therefore, the zwitterionic-grafted polymer layers attract water molecules only, and repel protein molecules.25 However, under loading conditions, water molecules in the PMPC layer would be squeezed out, and its hydration– lubrication effect and protein-adsorption suppression effect caused by the PMPC layer may be diminished. If water molecules and its thin film disappeared by applying vertical load, the proteins may physically adsorb onto the PMPC layer (maybe penetrating into the PMPC semi-brush layer). Then, the shear resistance of the PMPC-grafted CLPE surface increases. Thus, the damage of the PMPC layer would be accelerated in lubricants containing proteins. Here, the damage of PMPC layer was suppressed by rehydration and HA additives. Therefore, the results of this study indicated that sustaining the hydrated condition of the PMPC layer and the lubricating effects of HA are important for maximizing and maintaining the lubricity of the PMPC layer. However, the mechanism of accelerating the damage of the PMPC layer owing to adsorbed molecules is not yet clearly established. Therefore, adsorption behaviors of proteins, lipids, and surrounding-matrix macromolecules (e.g. proteoglycans, glycosaminoglycans, and HA) under frictional loading condition should be investigated in future studies. In previous hip and knee joint simulator studies, the PMPC-grafted CLPE shows extremely low friction and low wear.14–18 During a physical walking motion, referred to as the ISO 14242-3 standard, the applied load varies dramatically and loading and unloading are repeated within periods as short as 1 s. We think that this condition is beneficial to maintaining the hydration–lubrication effect of PMPC-grafted CLPE, thanks to the short duration of loading time. In contrast, the duration applied constant load in this study was 5 s for the preliminary examination, and this might not have provided a sufficient range of loading and motion conditions for physical walking or during daily routines. Overall, evaluation of lubrication characteristics of PMPC-grafted CLPE under the condition in which the applied load varies in shorter cycles will be needed for understanding the low-friction and highwear resistance mechanisms of PMPC-grafted CLPE. The evaluation of the effects of shorter cycles for loading–unloading is planned as our next step.
Conclusion In this study, we have evaluated the hydration– lubrication characteristics of PMPC-grafted CLPE by friction test with/without rehydration procedures in various lubricants. Rehydration by unloading is elemental in the lubrication effects of the hydrated PMPC layer. Additionally, the HA acts effectively in maintaining the PMPC layer. However, proteins and lipids have the potential to cause the failure of hydrated PMPC layer. Under physical walking, in which the applied
load varies dramatically and synovial fluid functions as a lubricant, the hydration lubrication of PMPC-grafted CLPE would act effectively and contribute to the reduction of friction of CLPE. Acknowledgements We thank Dr Yoshio Takatori of the University of Tokyo for valuable discussions and suggestions, and Dr Fumiaki Miyaji, Mr Kenichi Saiga, and Ms Shihori Yamane of KYOCERA Medical Corporation for their technical assistance. Declaration of conflicting interests Two of the authors (Masayuki Kyomoto and Kenichi Watanabe) are employed by KYOCERA Medical Corporation. One (Toru Moro) received outside funding from KYOCERA Medical Corporation. Funding Part of this research was supported by the Health and Welfare Research Grants for Research on Publicly Essential Drugs and Medical Devices (H23-007), Research on Measures for Intractable Diseases (H24001), and Research on Development of New Medical Devices (H26-006) from the Japanese Ministry of Health, Labour and Welfare.
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Influences of dehydration and rehydration on the lubrication properties of phospholipid polymergrafted cross-linked polyethylene
Proc IMechE Part H: J Engineering in Medicine 2015, Vol. 229(7) 506–514 Ó IMechE 2015 Reprints and permissions: sagepub.co.uk/journalsPermissions.nav DOI: 10.1177/0954411915588969 pih.sagepub.com
Seido Yarimitsu1,2, Toru Moro3,4, Masayuki Kyomoto3,5,6, Kenichi Watanabe3,6, Sakae Tanaka4, Kazuhiko Ishihara5 and Teruo Murakami1
Abstract 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 PMPCgrafted 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.
Keywords Artificial joint, MPC polymer, hydration lubrication, wear, friction
Date received: 24 September 2014; accepted: 5 May 2015
Introduction The number of total joint replacements is increasing every year.1 This type of surgery can relieve serious pain and improve quality of life. The most popular common bearing surface for artificial joints is a combination of an ultra-high molecular weight polyethylene (UHMWPE) and a cobalt–chromium–molybdenum (Co–Cr–Mo) alloy. The materials (i.e. cross-linked polyethylene (CLPE)) of artificial joints have been continually improved with the aim of reducing the UHMWPE wear particles. However, the three big complications limiting survivorship and clinical success,2–5 that is, dislocation, infection, and aseptic loosening, are not still solved completely. Hence, numerous attempts have been made to improve the wear resistance of the UHMWPE substrate. The cross-linking by gamma-ray irradiation is one of the effective treatments for reduction of wear of UHMWPE.6–8 Furthermore, vitamin E blending or diffusion is effective in preventing the
oxidative degradation and delamination wear of UHMWPE.9,10 However, those improvements to the UHMWPE substrate have not yet completely solved the wear-related problems. To prevent periprosthetic osteolysis and subsequent aseptic loosening, Moro et al.11 have studied the 1
Research Center for Advanced Biomechanics, Kyushu University, Fukuoka, Japan 2 Faculty of System Design, Tokyo Metropolitan University, Tokyo, Japan 3 Division of Science for Joint Reconstruction, Graduate School of Medicine, The University of Tokyo, Tokyo, Japan 4 Sensory & Motor System Medicine, Faculty of Medicine, The University of Tokyo, Tokyo, Japan 5 Department of Materials Engineering, School of Engineering, The University of Tokyo, Tokyo, Japan 6 Research Department, KYOCERA Medical Corporation, Osaka, Japan Corresponding author: Seido Yarimitsu, Faculty of System Design, Tokyo Metropolitan University, 6-6 Asahigaoka, Hino, Tokyo 191-0065, Japan. Email: [email protected]
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lubrication mechanism of natural synovial joint and the surface structure of natural articular cartilage. Natural articular cartilage has a hydration gel layer and phospholipid layer on its surface,12 and these surface layers contribute to the excellent lubrication property of natural synovial joint. Therefore, they have developed a new artificial joint system with a 100-nm-thick surface hydration layer formed by poly(2-methacryloyloxyethyl phosphorylcholine (MPC)) (PMPC)13 grafted onto the surface of CLPE (PMPC-grafted CLPE),11 mimicking the surface layer of natural articular cartilage.12 In previous hip and knee joint simulator studies, PMPCgrafted CLPE showed extremely low wear.14–17 Especially, the effect of PMPC grafting was maintained up to 70 million cycles in the hip simulator test.18 In addition, the PMPC-grafted particles were biologically inert and did not cause subsequent bone-resorptive responses.11 In the follow-up study of clinical trials of PMPC-grafted CLPE, no complication derived from PMPC-grafted CLPE occurred and the linear wear rate of PMPC-grafted CLPE was one-ordered smaller than other conventional CLPEs.19,20 Therefore, PMPCgrafted CLPE should contribute dramatically toward the improved longevity of artificial hip joint. The lubrication using a hydrated gel layer formed by hydrophilic polymer on the rubbing surface is important in maintaining low friction.21,22 In aqueous solution, the hydrophilic macromolecules on the bearing surfaces attract water molecules and form a water-rich hydrated gel layer. If a load is applied to the surface, it is mostly supported by the fluid pressure in the hydrated gel layer. In the case of sliding motion, water molecules are sheared with low stress, and low friction is maintained until the water is discharged by loading from the hydrated gel layer.23 This mechanism is known as hydration lubrication. Excellent lubricity and durability of the PMPC-grafted CLPE are exhibited, owing to this hydration lubrication. Therefore, it is important to elucidate the details pertaining to hydration lubrication of the PMPC layer in the bearing interface for further improvement in the longevity of artificial joints. In this study, we therefore investigated the effects of dehydration and rehydration by loading–unloading procedure on lubrication properties of PMPC-grafted CLPE using various lubricants. An in situ visualization technique was used in simplified reciprocating friction tests to track changes in the surface of PMPC-grafted CLPE. Throughout our studies, we questioned whether the hydration kinetics of the hydrated PMPC layer would affect the friction kinetics of the PMPC-grafted CLPE surface.
Germany) bar stock was irradiated with a 50-kGy dose of gamma-rays in a N2 gas, and annealed at 120 °C for 7.5 h in N2 gas in order to facilitate cross-linking. The resulting CLPE pin specimens were then machined from this bar stock after cooling. The pin geometry was a flat-ended, conical/cylindrical shape (u6 3 9 mm) with a tip diameter of u2 mm. The rubbing surfaces of CLPE specimens were polished and the arithmetical mean roughness (Ra) was approximately 0.4 mm.
Materials and methods CLPE pin specimen
In situ visualization on PMPC layers in friction tests with the rehydration procedure
A compression-molded UHMWPE (GUR1020 resin; Quadrant PHS Deutschland GmbH, Vreden,
A schematic illustration of the reciprocating friction tester used in this study is shown in Figure 1(a). The
PMPC-grafted CLPE pin specimen The CLPE pin specimens were immersed in acetone (Wako Pure Chemical Industries, Ltd, Osaka, Japan) containing 10 mg/mL benzophenone (Wako Pure Chemical Industries, Ltd) for 30 s, and then dried in the dark at room temperature in order to remove the acetone. Industrially synthesized MPC was purchased from NOF Corp. (Tokyo, Japan).13 The MPC was dissolved in degassed pure water to a concentration of 0.5 mol/L. Subsequently, the benzophenone-coated CLPE specimens were immersed in the MPC aqueous solutions. Photo-induced graft polymerization was carried out on the CLPE surface using ultra-violet (UV) irradiation (UVL-400HA ultra-high pressure mercury lamp; Riko-Kagaku Sangyo Co., Ltd, Funabashi, Japan) with an intensity of 5.0 mW/cm2 at 60 °C for 90 min; a filter (model D-35; Toshiba Corp., Tokyo, Japan) was used to restrict the passage of UV light to wavelengths of 350 6 50 nm.15 After the polymerization, the PMPC-grafted CLPE pin specimens were removed, washed with pure water and ethanol, and dried at room temperature. All obtained specimens were then sterilized by 25 kGy gamma-rays in a N2 gas.
Single-crystal sapphire A single-crystal sapphire plate (KYOCERA Corporation, Kyoto, Japan) was used as a counterface against the untreated and PMPC-grafted CLPE pins owing to observations of the frictional surface through the transparent sapphire plate during the friction test. The surface of the sapphire plate was mirror-finished, and its Ra was approximately 0.05 mm.
Fluorescent labeling of PMPC For in situ visualization of the PMPC layer, the PMPCgrafted layer was fluorescently labeled with rhodamine 6G (Wako Pure Chemical Industries, Ltd). The PMPCgrafted CLPE pin specimens were immersed in an aqueous solution of 200 mass ppm rhodamine 6 G for 30 s and then removed. These were washed twice in distilled water for 30 s and then dried.
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Proc IMechE Part H: J Engineering in Medicine 229(7) grafted CLPE pin specimens were soaked in each lubricant for 10 min for hydration of PMPC layer. The reciprocating test was initiated immediately after applying a load, and the reciprocating motion continued for five cycles (5 s). After the reciprocating cycles, the upper pin specimen was separated from the counterface and was kept with unloading for 60 s in a lubricant. Then, the friction test was restarted, and these procedures were repeated until a total sliding distance of 2.0 m was reached. At sliding distances of 0.05, 0.25, 0.50, 1.0, and 2.0 m, the PMPC layer on the sliding surface was imaged using fluorescence microscopy.
Statistical analyses The mean values for the friction coefficients of the four groups (untreated CLPE and PMPC-grafted CLPE with/without rehydration) were compared by a onefactor analysis of variance (ANOVA), and the significant differences (p \ 0.05) of the all comparable properties were determined by post hoc testing using Tukey’s method. All statistical analyses were performed using a KaleidaGraph (Synergy Software, Philadelphia, PA, USA).
Results In pure water
Figure 1. Reciprocating friction test for in situ visualization of a sliding surface: (a) details of the reciprocating friction tester and (b) schematic illustration of a reciprocating friction test with a rehydration procedure.
reciprocating friction tester was constructed on the stage of an inverted fluorescence microscope (IX-71, Olympus Corporation, Tokyo, Japan). Sliding pairs of the untreated or PMPC-grafted CLPE pin specimen and the sapphire plate were evaluated by using the reciprocating friction tester, and the sliding surface of the PMPC-grafted CLPE pin specimens was observed through the transparent sapphire plate. The reciprocating friction tests were performed at room temperature with a vertical load of 7.84 N (mean contact pressure was 2.50 MPa), a sliding speed of 10 mm/s, and a reciprocating stroke of 10 mm. The total sliding distance was 2.0 m. Four lubricants were used in this study: pure water, 0.5 mass% hyaluronic acid (HA; Mw = 9.2 3 105) aqueous solution, 30 vol.% fetal bovine serum (FBS; Invitrogen Corporation, Carlsbad, CA, USA) solution, and a mixture of 30 vol.% FBS and 0.5 mass% HA. A schematic illustration of the reciprocating friction test with the rehydration procedure is shown in Figure 1(b). Before the reciprocating test, the PMPC-
The friction coefficients of each of the untreated and PMPC-grafted CLPE in pure water are shown in Figure 2. The start-up friction of PMPC-grafted CLPE was significantly lower than that of CLPE, regardless of rehydration (Figure 2(a) and (b)). With the rehydration procedures, the PMPC-grafted CLPE maintained a lower friction during the 2.0-m test (Figure 2(a) and (c)). The recovery and gradual increase in friction coefficient for each rehydration procedure was observed both in cases of untreated and PMPC-grafted CLPE. In contrast, the friction coefficient of PMPC-grafted CLPE without rehydration was nearly identical with that of CLPE. The fluorescence microscopy images of the PMPC layer are shown in Figure 3. With rehydration procedure, damage of the PMPC layer progressed partially in the convex region of the machine marks (arc-like patterns) on the PMPC-grafted CLPE surface, with increasing sliding distance. In contrast, the PMPC layer on the convex region of the machine marks was severely damaged without rehydration procedures.
In HA solution When HA solution was used as lubricant, the start-up friction of PMPC-grafted CLPE was lower than that of CLPE, regardless of rehydration (Figure 4(a) and (b)). However, there was no significant difference in the friction coefficient at steady state between untreated and PMPC-grafted CLPE (Figure 4(c)). Friction of the PMPC-grafted CLPE recovered slightly with each of
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Figure 2. Friction coefficient in pure water: (a) transient of friction coefficient, (b) friction coefficient at the 1st cycle in the reciprocating friction test, and (c) friction coefficient at a sliding distance of 2.0 m. Values: mean 6 standard deviation (n = 5). *p \ 0.05 (Tukey’s HSD test).
Figure 3. Fluorescence microscopy images of the PMPC layer lubricated in pure water (numbers in the subcaption indicate sliding distance, bars: 50 mm, arrows: sliding direction): (a) 0.05 m, (b) 0.25 m, (c) 0.50 m, (d) 1.00 m, (e) 2.00 m, and (f) 2.00 m.
the rehydration procedures, but there was no significant difference in the friction coefficient of the PMPCgrafted CLPE in cases with and without rehydration. The fluorescence microscopy images of the PMPC layer lubricated in the HA solution are shown in Figure 5. The damage of the PMPC layer in HA solution was slight, despite the lack of rehydration procedure (Figure 5(f)), and the suppression of damage of the PMPC layer was confirmed when rehydration procedures were provided, even after 2.0 m sliding tests.
In FBS solution Figure 6 shows the friction coefficient of the untreated and PMPC-grafted CLPE in 30 vol.% FBS solution. The start-up friction of PMPC-grafted CLPE was lower than that of CLPE (Figure 6(a) and (b)). As shown in Figure 6(a) and (c), the PMPC-grafted CLPE with
rehydration maintained a lower friction coefficient than that of untreated CLPE with/without rehydration and PMPC-grafted CLPE without rehydration. The friction coefficient of PMPC-grafted CLPE without rehydration gradually increased and approached asymptotically to that of CLPE. The damage of the PMPC layer was limited to the convex region of the machine marks on the surface of PMPC-grafted CLPE when rehydration procedures were performed (Figure 7). In contrast, in the case without rehydration, damage of the PMPC layer ranged from the convex to concave regions of the machine marks on the PMPC-grafted CLPE surface.
In a mixture of FBS and HA solution Figure 8 shows the friction coefficient of the untreated and PMPC-grafted CLPE in a mixture of 30 vol.% FBS and 0.5 mass% HA. Both the start-up and steady-
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Figure 4. Friction coefficient in HA 0.5 mass% solution: (a) transient of friction coefficient, (b) friction coefficient at the 1st cycle in the reciprocating friction test, and (c) friction coefficient at a sliding distance of 2.0 m. Values: mean 6 standard deviation (n = 5). *p \ 0.05 (Tukey’s HSD test).
Figure 5. Fluorescence microscopy images of the PMPC layer lubricated in HA 0.5 mass% solution (numbers in the subcaption indicate sliding distance, bars: 50 mm, arrows: sliding direction): (a) 0.05 m, (b) 0.25 m, (c) 0.50 m, (d) 1.00 m, (e) 2.00 m, and (f) 2.00 m.
state friction of the PMPC-grafted CLPE was lower than that of CLPE, regardless of rehydration. The friction coefficient of untreated and PMPC-grafted CLPE rose and then gradually decreased during each rehydration. There was no difference in the friction coefficient of the PMPC-grafted CLPE for cases with and without hydration. Damages of the PMPC layer on the convex region of the machine marks on the PMPC-grafted CLPE surface were suppressed with rehydration (Figure 9).
Discussion In this study, we investigated the effects of dehydration and rehydration by loading–unloading procedure on the lubrication properties of PMPC-grafted CLPE in
various lubricants by using an in situ visualization technique with simplified reciprocating friction. At the beginning of this study, we questioned whether the hydration kinetics of a hydrated PMPC layer would affect the friction kinetics of a PMPC-grafted CLPE surface. The friction properties of PMPC-grafted CLPE, in terms of the rehydration effects of the PMPC layer, will be discussed hereafter. The hydrated PMPC layer clearly affected the friction response; the friction coefficients for the start-up motion of the PMPC-grafted CLPE surfaces were significantly lower than those of the untreated CLPE surface with every lubricant tested. This behavior is attributed to the significant increase in hydrophilicity that is evident from the reduction in the static watercontact angles of the PMPC-grafted surfaces.11,14
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Figure 6. Friction coefficient in FBS 30 vol.% solution: (a) transient of friction coefficient, (b) friction coefficient at the 1st cycle in the reciprocating friction test, and (c) friction coefficient at a sliding distance of 2.0 m. Values: mean 6 standard deviation (n = 5). *p \ 0.05 (Tukey’s HSD test).
Figure 7. Fluorescence microscopy images of the PMPC layer lubricated in FBS 30 vol.% solution (numbers in the subcaption indicate sliding distance, bars: 50 mm, arrows: sliding direction): (a) 0.05 m, (b) 0.25 m, (c) 0.50 m, (d) 1.00 m, (e) 2.00 m, and (f) 2.00 m.
The reduction of the friction coefficient of PMPCgrafted CLPE by rehydration in the steady state was observed only in pure water and in HA solution. These results may imply that the lubrication of PMPC-grafted CLPE was dominated by the hydration–lubrication mechanism. However, the PMPC layer gradually lost its effect under continuous loading. When the load is applied continuously or for long periods, water molecules in the PMPC layer are squeezed out. Thus, the effects of hydration lubrication may be diminished, and dehydration likely leads to an increase in friction as well as damage of the PMPC layer as a result. Therefore, ensuring rehydration by unloading or load variation is essential in order to establish and maintain the effect of hydration lubrication of the PMPC layer. The friction coefficient in HA solution was higher than
that in pure water, but the damage of PMPC layer was suppressed by addition of HA. It was thought that the increase in friction by HA additives was attributed to the viscous resistance of HA solution and/or adsorbed HA layer. However, HA would contribute to the separation of frictional mating surfaces by enhancement of fluid film formation or formation of adsorbed film. The untreated CLPE, lubricated in water or HA solution, also showed significant reduction in its friction coefficient after unloading despite having no hydration layer on its surface. This is because the conformity of a bearing interface is improved by creep deformation of the CLPE substrate. At the initial state including start-up of the friction test, the friction coefficients of both untreated and PMPC-grafted CLPE decreased gradually with an increase in sliding distance.
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Figure 8. Friction coefficient in a mixture of FBS 30 vol.% and HA 0.5 mass% solution: (a) transient of friction coefficient, (b) friction coefficient at the 1st cycle in the reciprocating friction test, and (c) friction coefficient at a sliding distance of 2.0 m. Values: mean 6 standard deviation (n = 5). *p \ 0.05 (Tukey’s HSD test).
Figure 9. Fluorescence microscopy images of the PMPC layer lubricated in a mixture of FBS 30 vol.% and HA 0.5 mass% solution (numbers in the subcaption indicate sliding distance, bars: 50 mm, arrows: sliding direction): (a) 0.05 m, (b) 0.25 m, (c) 0.50 m, (d) 1.00 m, (e) 2.00 m, and (f) 2.00 m.
In the hydration–lubrication model, it is well known that the friction coefficient gradually increases after applying load and reaches plateau.22 Hence, it may be the case that the running-in effect of the surfaces of PMPC-grafted CLPE and CLPE overshadows the effect of hydration lubrication during the initial-state friction test. In FBS solution, an increasing of friction coefficient after rehydration was clearly observed. This was the characteristic phenomenon for FBS solution. It was expected that an adsorbed film would be formed on the surface of untreated and PMPC-grafted CLPE since FBS contains several proteins (e.g. albumin and globulin) and lipids. After the boundary films were formed through rubbing and the applied load was removed,
further proteins and lipids would adsorb on the rubbed surfaces. Immediately after reloading, the excessively adsorbed molecules may interact with high friction, but during continuous rubbing, the boundary films appear to be reoriented by frictional force. It may be the case that a decrease in friction coefficient after restarting frictional motion is due to this reorientation of adsorbed film by rubbing. In the FBS solution, the damage of PMPC layer was accelerated compared with that of water or HA lubricants. It is well established that a hydrated PMPC layer prevents protein adsorption because of zwitterionic polyelectrolyte.24 Moreover, the zeta potential of the PMPC-grafted CLPE surface was close to zero because the ionic group in the MPC unit forms an inner salt and the
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electrostatic effects are diminished. Therefore, the zwitterionic-grafted polymer layers attract water molecules only, and repel protein molecules.25 However, under loading conditions, water molecules in the PMPC layer would be squeezed out, and its hydration– lubrication effect and protein-adsorption suppression effect caused by the PMPC layer may be diminished. If water molecules and its thin film disappeared by applying vertical load, the proteins may physically adsorb onto the PMPC layer (maybe penetrating into the PMPC semi-brush layer). Then, the shear resistance of the PMPC-grafted CLPE surface increases. Thus, the damage of the PMPC layer would be accelerated in lubricants containing proteins. Here, the damage of PMPC layer was suppressed by rehydration and HA additives. Therefore, the results of this study indicated that sustaining the hydrated condition of the PMPC layer and the lubricating effects of HA are important for maximizing and maintaining the lubricity of the PMPC layer. However, the mechanism of accelerating the damage of the PMPC layer owing to adsorbed molecules is not yet clearly established. Therefore, adsorption behaviors of proteins, lipids, and surrounding-matrix macromolecules (e.g. proteoglycans, glycosaminoglycans, and HA) under frictional loading condition should be investigated in future studies. In previous hip and knee joint simulator studies, the PMPC-grafted CLPE shows extremely low friction and low wear.14–18 During a physical walking motion, referred to as the ISO 14242-3 standard, the applied load varies dramatically and loading and unloading are repeated within periods as short as 1 s. We think that this condition is beneficial to maintaining the hydration–lubrication effect of PMPC-grafted CLPE, thanks to the short duration of loading time. In contrast, the duration applied constant load in this study was 5 s for the preliminary examination, and this might not have provided a sufficient range of loading and motion conditions for physical walking or during daily routines. Overall, evaluation of lubrication characteristics of PMPC-grafted CLPE under the condition in which the applied load varies in shorter cycles will be needed for understanding the low-friction and highwear resistance mechanisms of PMPC-grafted CLPE. The evaluation of the effects of shorter cycles for loading–unloading is planned as our next step.
Conclusion In this study, we have evaluated the hydration– lubrication characteristics of PMPC-grafted CLPE by friction test with/without rehydration procedures in various lubricants. Rehydration by unloading is elemental in the lubrication effects of the hydrated PMPC layer. Additionally, the HA acts effectively in maintaining the PMPC layer. However, proteins and lipids have the potential to cause the failure of hydrated PMPC layer. Under physical walking, in which the applied
load varies dramatically and synovial fluid functions as a lubricant, the hydration lubrication of PMPC-grafted CLPE would act effectively and contribute to the reduction of friction of CLPE. Acknowledgements We thank Dr Yoshio Takatori of the University of Tokyo for valuable discussions and suggestions, and Dr Fumiaki Miyaji, Mr Kenichi Saiga, and Ms Shihori Yamane of KYOCERA Medical Corporation for their technical assistance. Declaration of conflicting interests Two of the authors (Masayuki Kyomoto and Kenichi Watanabe) are employed by KYOCERA Medical Corporation. One (Toru Moro) received outside funding from KYOCERA Medical Corporation. Funding Part of this research was supported by the Health and Welfare Research Grants for Research on Publicly Essential Drugs and Medical Devices (H23-007), Research on Measures for Intractable Diseases (H24001), and Research on Development of New Medical Devices (H26-006) from the Japanese Ministry of Health, Labour and Welfare.
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