- Email: [email protected]
Influence of protein and lipid concentration of the test lubricant on the wear of ultra high molecular weight polyethylene
ARTICLE IN PRESS
Tribology International 41 (2008) 648–656 www.elsevier.com/locate/triboint
Influence of protein and lipid concentration of the test lubricant on the wear of ultra high molecular weight polyethylene Y. Sawaea,, A. Yamamotob, T. Murakamia a
Department of Intelligent Machinery and Systems, Faculty of Engineering, Kyushu University, 744 Motooka, Nishi-ku, Fukuoka 819-0395, Japan b Graduate School of Engineering, Kyushu University, 744 Motooka, Nishi-ku, Fukuoka 819-0395, Japan Received 9 February 2007; received in revised form 11 November 2007; accepted 17 November 2007 Available online 2 January 2008
Abstract To gain a better understanding of the ultra-high molecular weight polyethylene (UHMWPE) wear mechanism in the physiological environment, the effects of protein and lipid constituents of synovial fluid on the specific wear rate of UHMWPE were examined experimentally. The multidirectional sliding pin-on-plate wear tester was employed to simulate the simplified sliding condition of hip joint prostheses. Bovine serum g-globulin and synthetic L-a-DPPC were used as model protein and lipid constituents of synovia, respectively. Results of the wear test indicated that the UHMWPE wear rate primarily depended on the protein concentration of the test lubricant. Lipids acted as a boundary lubricant and reduced polyethylene wear in the low protein lubricants. However, the polyethylene wear rate increased with increasing lipid concentrations if the protein concentration was within the physiological level. Increased interactions between protein and lipid molecules and lipid diffusion to polyethylene surface might be responsible for the increased wear. r 2007 Elsevier Ltd. All rights reserved. Keywords: UHMWPE; Wear; Joint prosthesis; Protein; Phospholipid
1. Introduction Although various material combinations are used in the bearing surface of current joint prostheses, ultra-high molecular weight polyethylene (UHMWPE) is still the most popular bearing material for artificial joints. UHMWPE was first introduced in the bearing surface of hip joint prosthesis by Prof. Charnley since it seemed to have good tribological characteristics, low friction and low wear, and good biocompatibility as well. However, it has become evident that fine wear particles released from the UHMWPE component cause serious adverse reactions within the human body. They activate macrophages and cause necrosis of surrounding bone tissues. These histological changes finally induce the aseptic loosening of joint components [1,2]. Therefore, the wear characteristic of the UHMWPE is now recognized as determining factor of the
Corresponding author. Tel.: +81 92 802 3073; fax: +81 92 802 0001.
E-mail address: [email protected] (Y. Sawae). 0301-679X/$ - see front matter r 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.triboint.2007.11.010
durability and the long-term survivorship of artificial joint prostheses. Many attempts have been made to improve the wear property of UHMWPE [3–8]. The wear characteristics of newly developed materials should be examined in laboratory wear tests before clinical use. In in vitro wear estimations, the appropriate understanding of the in vivo wear mechanism is essential to reproduce the physiological wear behaviour and to obtain clinically reliable wear test results. However, the UHMWPE wear process in the human body is complicated, since joint prostheses are lubricated with synovia, which is a kind of body fluid secreted into the joint capsule and has a complicated composition including many kinds of electrolytes, radicals and biological macromolecules. These constituents are entrained into the contact zone during joint movement and might affect the friction and wear behaviour through chemical and mechanical interactions. Some ex vivo studies have suggested the considerable effects of the physiological environment on the material properties and subsequent wear behaviour of the implanted
ARTICLE IN PRESS Y. Sawae et al. / Tribology International 41 (2008) 648–656
UHMWPE. Eyerer and co-workers [9,10] indicated the accelerated oxidative degradation of the polyethylene articulating surface caused by the peroxide constituents of synovial fluid. The enhanced oxidation and subsequent mechanical property changes in the polyethylene surface were also demonstrated by Costa et al. [11] and Ambrosio et al. [12]. They described these adverse effects of the physiological environment as ‘‘biodegradation’’ and ‘‘biological corrosion’’, respectively. The influence of biological macromolecules contained in the synovial fluid has been examined by in vitro experimental methods. Specifically, the effects of serum proteins, the primal macromolecule constituent of synovial, on the wear behaviour of UHMWPE were examined by several research groups [13–19]. Wang et al. [13] focused on the protein concentration of test lubricants used in their simulator test and reported that the wear rate of the serum lubricated UHMWPE cup clearly increased by increasing the protein concentration of the lubricant. Similar results were also reported by Saikko [14] by using his circularly translating pin-on-disk (CTPOD) device [20]. Wang et al. [15] also indicated the effect of protein type and their relative concentration in the lubricant. Phospholipids are another macromolecule constituent of the synovial fluid, and its role as a boundary lubricant in the natural synovial joints has been demonstrated by Hills [21,22] and other researchers [23]. In contrast to proteins, there are a limited number of reports about the influence of phospholipid molecules on the tribological behaviour of prosthetic joint materials. However, Mazzucco et al. reported that the phospholipid concentration in synovial fluid obtained from joints implanted with joint prostheses is at the same level as that obtained from diseased joints [24], and several ex vivo analyses of retrieved polyethylene components indicate that the lipid constituents of synovia can diffuse into UHMWPE in vivo [25,26]. Greenbaum et al. [27] examined the effects of diffused lipid molecules on the mechanical properties and wear characteristics of UHMWPE experimentally. Their test results indicated that the lipid molecules that diffused into the bulk of the material had a significant plasticising effect on UHMWPE. Although the apparent effect of lipid molecules on the polyethylene wear behaviour could not be identified under their test conditions, the material property change in the bearing surface could alter the contact condition that might affect the material deformation in the real contact area and subsequent tribological conditions. Therefore, effects of lipid molecules on friction and wear should be postulated carefully. The final goal of this study is to understand the in vivo wear mechanism of UHMWPE and to identify the critical factors determining the wear behaviour of UHMWPE in a physiological environment. In the current study, we focused on biological macromolecules, specifically protein and phospholipid molecules, contained in synovia and experimentally examined their influence on the UHMWPE wear.
649
2. Materials and methods 2.1. Multidirectional pin-on-plate wear test The wear behaviour of UHMWPE sliding against medical grade anti-corrosive metal was evaluated by using a three-station multidirectional sliding pin-on-plate wear tester. This wear test apparatus was developed in our laboratory to simulate the polyethylene wear in hip joint by the simplified multidirectional sliding motion [28]. The basic principle of the multidirectional sliding in the present wear tester is identical to that of the CTPOD device developed by Saikko [20]. Fig. 1 is a schematic drawing of the multidirectional sliding wear tester used in this study. Three pin specimens were attached to loading bars and loaded individually by dead weights against a mated plate specimen. Each plate specimen was installed in a separate liquid bath and mounted on the X–Y stage which was moved by a motor and crank mechanism. A sliding motion arising between a pin and plate specimen in this apparatus is shown schematically in Fig. 2. In this case, the fixed pin specimen articulates upon the plate specimen surface along a circular path with a diameter of 30 mm. Then, the velocity vector is varied from v to v0 while the centre of the plate specimen moves from o to o0 . As a result, the direction of the sliding velocity vector changes continuously relative to the UHMWPE pin surface whereas the magnitude of the velocity vector remains constant at the any point on the pin surface. Cylindrical polyethylene pin specimens with a diameter of 6 mm were cut from GUR415 bar stock and were not
Fig. 1. Three-station multidirectional sliding pin-on-plate wear tester.
ARTICLE IN PRESS 650
Y. Sawae et al. / Tribology International 41 (2008) 648–656
Fig. 2. Schematic drawing of multidirectional sliding motion between pin and plate specimen.
sterilized prior to the wear tests. Cast Co–Cr–Mo alloy (ASTM-F75) was used as a plate specimen. The plate surface was polished by diamond slurry and the resultant surface roughness was measured by a stylus roughness meter. The average and the standard deviation of measured Ra values were 0.009 and 0.005 mm, respectively. Prior to the wear test, both pin and plate specimens were washed in an ultrasonic cleaner with a detergent (polyoxyethylene p-toctylphenylether) for 30 min. Then the specimens were rinsed in a stream of distilled water and washed in the ultrasonic cleaner again for 30 min with distilled water. Finally, the specimens were washed ultrasonically in ethanol for 15 min and dried in a vacuum desiccator. Wear tests were conducted at room temperature. A mean contact pressure of 7 MPa and a magnitude of the sliding velocity vector of 20 mm/s were applied between pin and plate specimens. Each test was run for a sliding distance of 10 km. The wear amount of the pin specimen was determined from the weight loss. At intervals of 5 km sliding, the pin specimen was removed from the test apparatus and cleaned by the same procedure described before. Then the specimen was dried in a vacuum desiccator for 2 h at 50 1C and weighed to an accuracy of 0.01 mg. A soak control pin was prepared to estimate the weight gain due to fluid uptake from the lubricant. Then, the specific wear rate, defined as the wear volume per unit load and unit sliding distance, was calculated from the weight loss of the UHMWPE pin specimens. After the wear test, morphological characteristics of the polyethylene worn surface were examined by optical microscopy. Furthermore, microscopic and nanoscopic structures of the worn surface were observed by atomic force microscopy (AFM) to examine the wear mechanism of UHMWPE. 2.2. Test lubricant Several phosphate buffered saline (PBS) solutions containing protein and lipid were prepared and used in
the wear test as lubricants. In this study, bovine serum g-globulin was chosen as a representative protein contained in synovia since it exhibited a more significant effect compared with albumin in our previous study [28,29]. Synthetic phospholipid (L-a-dipalmitoyl phosphatidylcholine, DPPC) was used as an alternative material for lipid constituents of synovial fluid. Typical preparation method of liposome was employed to disperse DPPC molecules in PBS as fine colloidal particles. In this method, an appropriate amount of DPPC powder was dissolved in chloroform. The DPPC solution was then poured into a flask and the solvent was subsequently removed by a rotary evaporator yielding a thin lipid film on the flask surface. The resultant lipid thin film was hydrated with PBS and finally ultrasonicated to disperse the DPPC vesicles homogeneously in PBS. Subsequently, g-globulin was dissolved in the DPPC dispersion. A 0.3 wt% sodium azide was also added to prevent bacterial growth. In this study, test lubricants with different compositions were prepared, and the effects of protein and lipid concentrations of the lubricant on UHMWPE wear were examined. The protein concentration was varied from a relatively small value of 0.1 wt% to physiological protein concentrations of synovial fluid, up to 2.0 wt% [23,24]. PBS solutions containing physiological amounts of protein molecules, 1.0 and 2.0 wt%, were defined as high protein lubricants, while the solutions with a protein concentration of 0.1, 0.2 and 0.4 wt% were categorized as low protein lubricants. On the other hand, the lipid concentrations of test lubricants used in this study were varied within the physiological range of synovial phospholipid concentration, 0.005, 0.01 and 0.02 wt% [23,24]. Diluted bovine serum (30 vol% fetal bovine serum+70 vol% distilled water) and PBS containing no macromolecules were used as control lubricants. The diluted bovine serum used in this study has a protein concentration of 1.1 wt% consisting of an albumin content of 0.7 wt% and a globulin content of 0.4 wt%. 3. Results Figs. 3–5 show the average weight loss of UHMWPE pin specimens as a function of the sliding distance. Fig. 3 is the comparison between the diluted bovine serum and the macromolecule-free PBS. The wear amount of UHMWPE pin specimens linearly increased with the sliding distance in the diluted bovine serum while the UHMWPE wear was almost negligible in the macromolecule-free PBS. Wear behaviours of UHMWPE pin specimens in the low protein lubricants with globulin concentration of 0.2 wt% were shown in Fig. 4. In this case, the wear amount increased almost linearly in lubricants with DPPC concentration of 0.005 and 0.01 wt%. However, polyethylene wear was suppressed after 5 km sliding if the DPPC concentration was increased to 0.02 wt%. The wear behaviour in high protein lubricants with globulin concentration of 1.0 wt%
ARTICLE IN PRESS Y. Sawae et al. / Tribology International 41 (2008) 648–656
651
Fig. 3. Mean wear amount of UHMWPE pin specimens lubricated with 30 vol% bovine serum solution and macromolecular-free PBS plotted against sliding distance. N ¼ 3 for the serum solution and N ¼ 1 for PBS.
Fig. 6. Comparison of UHMWPE specific wear rate between 30 vol% bovine serum solution and macromolecular-free PBS. Graph indicates mean value of pin specimens. Error bar means minimum and maximum value. N ¼ 3 for the serum solution and N ¼ 1 for PBS. Fig. 4. Mean wear amount of UHMWPE pin specimens lubricated with the low protein lubricant (g-globulin 0.2 wt%) plotted against sliding distance. N ¼ 3 for DPPC 0.005 and 0.01 wt% and N ¼ 2 for DPPC 0.02.
Fig. 5. Mean wear amount of UHMWPE pin specimens lubricated with the high protein lubricant (g-globulin 1.0 wt%) plotted against sliding distance. N ¼ 3 for all lubricants.
was compared in Fig. 5. In this case, the polyethylene wear amount increased linearly in all lubricants, and the wear amount increased with increasing phospholipid concentrations. The wear amount of UHMWPE pin specimens increased linearly during 10 km sliding and the transition from the initial wear to the steady state wear could not be identified in most cases. Therefore, all wear amount obtained during
Fig. 7. Effect of phopholipid concentration on specific wear rate of UHMWPE in low protein lubricants. Graph indicates mean value of pin specimens. Error bar means minimum and maximum value. N ¼ 3 except all g-globulin 0.1 wt% lubricants and g-globulin 0.2 wt% lubricant with DPPC 0.02 wt%, for which N ¼ 2.
10 km sliding was included in the calculation of the specific wear rate. The specific wear rates of UHMWPE in the 30 vol% serum solution and the macromolecule-free PBS were shown in Fig. 6. The average value of specific wear rate was found to be 3.47 107 mm3/Nm in diluted bovine serum. On the other hand, the wear rate in
ARTICLE IN PRESS 652
Y. Sawae et al. / Tribology International 41 (2008) 648–656
macromolecular-free PBS was 0.05 107 mm3/Nm, which is about one hundredth of that in the serum solution. The UHMWPE wear rates in low protein lubricants are compared in Fig. 7. Polyethylene wear rates were significantly small in low protein solutions compared with that in the serum solution. In lubricants containing only 0.1 wt% globulin, the average wear rates were 0.48 107, 0.66 107 and 0.18 107 mm3/Nm for the DPPC concentration of 0.005, 0.01 and 0.02 wt%, respectively. Then, the UHMWPE wear rate gradually increased to
Fig. 8. Effect of phopholipid concentration on specific wear rate of UHMWPE in high protein lubricants. Graph indicates mean value of pin specimens. Error bar means minimum and maximum value. N ¼ 3 for all lubricants.
1.06 107 and 1.89 107 mm3/Nm with increasing globulin concentration to 0.2 and 0.4 wt%, respectively, with the lowest DPPC concentration of 0.005 wt%. However, the wear rate in the lubricants containing 0.4 wt% globulin could be reduced to 0.78 107 and 0.52 107 mm3/Nm by increasing DPPC concentration to 0.01 and 0.02 wt%, respectively. UHMWPE wear rates in the high protein lubricants were shown in Fig. 8. The polyethylene wear rates in the high protein lubricants were apparently higher than those in the low protein lubricants, and the wear rate clearly increased with increasing lipid concentrations in this case. The average wear rate increased gradually from 2.30 107 to 3.03 107 and 3.52 107 mm3/Nm in the 1.0 wt% globulin solutions by increasing DPPC concentration from 0.005 to 0.01 and 0.02 wt%, respectively. Similar results were also obtained in the lubricants containing 2.0 wt% globulin. In this case, the average wear rate increased from 2.06 107 to 3.50 107 and 3.74 107 mm3/Nm by increasing the DPPC concentration. The average values of the UHMWPE wear rate are summarized and plotted against the protein concentration of test lubricants in Fig. 9. The wear rates in the diluted bovine serum and PBS were also plotted in the same figure. The UHMWPE wear rate primarily depended on the protein concentration of the lubricant and increased with increasing protein concentrations. However, the relationship between the polyethylene wear rate and the protein concentration of the test lubricant highly depended on the added phospholipid concentration. When the lubricant has a low phospholipid concentration of 0.005 wt%, the wear rate increased logarithmically with protein concentration and peaked at a protein concentration of 1.0 wt%. On the other hand, the increase in the polyethylene wear rate was limited within low protein regime if phospholipid concentration increased to 0.01 wt%. In this case, the polyethylene wear rate subsequently increased steeply by increasing protein concentration to 1.0 wt%, which is in a range of
Fig. 9. Average of UHMWPE wear rate plotted against protein concentration of lubricant.
ARTICLE IN PRESS Y. Sawae et al. / Tribology International 41 (2008) 648–656
653
Fig. 10. Comparison of optical micrographs from worn surface of UHMWPE pin specimens. (a) PBS, (b) g-globuline 1.0 wt%+DPPC 0.005 wt%, (c) g-globuline 1.0 wt%+DPPC 0.02 wt% and (d) g-globuline 2.0 wt%+DPPC 0.02 wt%.
physiological protein concentration of synovia, and became comparable to that in the diluted bovine serum. Optical micrographs from polyethylene worn surfaces are compared in Fig. 10. The initial surface of a pin specimen was covered with machinemarks, which formed during the specimen fabrication process, and many of them remained on the polyethylene worn surface in the macromolecular-free PBS (Fig. 10(a)). The visible machinemarks decreased as protein and phospholipid were added to PBS (Fig. 10(b)), and they were completely worn out if the globulin concentration and DPPC concentration of the lubricant were increased to 0.1 and 0.02 wt%, respectively (Fig. 10(c)). In this case, small protuberances could be found on the polyethylene worn surface. The polyethylene worn surface finally became very smooth in the lubricant containing 0.2 wt% globulin and 0.02 wt% DPPC. AFM images of the polyethylene worn surface are also compared in Fig. 11. Apparent differences in microscopic morphologies were observed between the macromolecular-free PBS and test lubricants containing protein and phospholipid; the polyethylene worn surfaces were covered with small asperities in test lubricants (Fig. 10(b–d)), although they were not observed on the worn surface in the macromolecular-free PBS (Fig. 10(a)). The size of asperities increased with increasing phospholipid concentration and fine fibrillated structures could be observed in the lubricant containing 2.0 wt% globulin and 0.02 wt% DPPC.
4. Discussion First, the polyethylene wear rate in macromolecule-free PBS was compared with the wear rate in the diluted bovine serum. The results imply the crucial effect of the macromolecules contained in bovine serum, since the wear amount of UHMWPE in PBS, which contains only several kinds of electrolytes, was almost negligible compared with that in the serum solution. This result is consistent with the results from the hip joint simulator test reported by Wang et al. [13]. In the current study, protein and phospholipid molecules were subsequently added to the PBS and their concentrations were changed to identify the influence of each constituent on the polyethylene wear behaviour. The specific wear rate of UHMWPE primarily depended on the protein concentration of the test lubricant. It gradually increased with increasing protein concentration of the lubricant. Similar results were also reported by Wang et al. [13] and Saikko [14]. They used pure water or distilled water as a control lubricant and the protein concentration of their test lubricant was adjusted by diluting bovine serum. On the other hand, the protein concentration of the test lubricant was regulated by adding serum proteins to PBS in this study. AFM observation revealed that the microscopic morphology of the polyethylene worn surface was significantly altered by protein molecules added to PBS. Fine asperities formed on the
ARTICLE IN PRESS 654
Y. Sawae et al. / Tribology International 41 (2008) 648–656
Fig. 11. Microscopic morphology of polyethylene worn surfaces observed by AFM. (a) PBS, (b) g-globuline 1.0 wt%+DPPC 0.005 wt%, (c) g-globuline 1.0 wt%+DPPC 0.02 wt% and (d) g-globuline 2.0 wt%+DPPC 0.02 wt%.
worn surface suggest the increased plastic deformation of the polyethylene surface in the lubricant containing protein molecules. Heuberger et al. [19] suggested that the protein molecules in lubricants could be denatured during a sliding test and unfolded proteins preferably adsorbed onto hydrophobic polyethylene surface. The adsorbed proteins might increase adhesion friction which enhanced microscopic plastic deformation of polyethylene surface and finally brought some changes in the polyethylene wear behaviour. The phospholipid concentration also had certain effects on the polyethylene wear rate in our experiments. However, the relationship between the phospholipid concentration of the lubricant and the wear rate of UHMWPE was somewhat complicated. The wear rate of UHMWPE was reduced by increasing the DPPC concentration of the test lubricant, if the protein concentration of the lubricant was lower than the physiological value for synovia, which is 20–40 mg/ml [13,14,23,24]. The polyethylene wear reduction in the low protein lubricant might suggest the potential of the phospholipid molecules as a boundary lubricant for
the prosthetic joint materials. The supplemented DPPC molecules might successfully form a boundary lubrication film on the material surfaces in the low protein lubricant and reduce polyethylene wear. The boundary lubrication effect of phospholipid for the artificial joint was also reported by Bell et al. [30], although the phospholipid concentration of their lubricants was higher than the physiological lipid concentration of synovial fluid. However, the boundary lubrication ability of the phospholipid molecules was suppressed by increasing the protein concentration to the physiological range, and inversely, the wear rate of UHMWPE increased by increasing the lipid concentration of the lubricant. The increased interaction between protein and phospholipid molecules is one of the possible reasons for the decrease in lipid boundary lubrication. Since lipid molecules can easily bind to proteins to form lipoproteins, the characteristics of the adsorbed boundary film may be altered by increased protein molecules. As a result, the adsorbed boundary film may lose its functional structure and lubrication abilities.
ARTICLE IN PRESS Y. Sawae et al. / Tribology International 41 (2008) 648–656
In addition to the protein–lipid interaction, there should be some interactions between phospholipid molecules and bearing surface materials. Several ex vivo analyses of retrieved polyethylene acetabular cups revealed that the lipid constituents of synovial fluid adsorbed on the polyethylene component and diffused into the bulk material in vivo [25,26], while evidence of the protein adsorption was observed only on the polyethylene surface [12,26]. Greenbaum et al. [27] examined the effect of diffused lipid molecules on the material properties of UHMWPE experimentally. They doped UHMWPE with cholestene and squalene and evaluate the changes in the compressive modulus and compressive strength of UHMWPE. Their experimental results clearly demonstrated the plasticizing effect of diffused lipid molecules since the compressive modulus and the compressive strength of UHMWPE significantly decreased after the lipid diffusion. These property changes might be responsible for the difference in the microscopic morphology of polyethylene worn surfaces observed in this study. AFM images indicated that the size and interval of small asperities, which formed on the polyethylene surface during sliding tests, became larger as the lipid concentration of the test lubricant increased. These results may suggest the plasticizing effect of phospholipid molecules added to the lubricant, which possibly alters the extent of microscopic plastic deformation in the polyethylene surfaces and consequently affect polyethylene wear behaviour. Furthermore, fine fibrils of the elongated polymer could be observed on the polyethylene worn surface only if the lubricant had high protein and phospholipid concentrations. As reported previously, a fine fibrillated microstructure is one of the common morphological characteristics observed in the load bearing area of retrieved polyethylene cups [31–33]. Wang et al. pointed out that the direction of the sliding motion in the laboratory wear test should be changed relative to the polyethylene surface during the experiment to reproduce the in vivo wear behaviour of UHMWPE [32]. The importance of the multidirectional sliding motion in the UHMWPE wear estimation was subsequently confirmed by other research groups [20,34–36]. Therefore, the multidirectional sliding pin-on-plate wear tester was employed to evaluate UHMWPE wear in this study. However, the worn surface morphologies observed by optical and atomic force microscopes were not necessarily in common with those found in retrieved polyethylene cups. In this study, wear tests were conducted with an average contact pressure of 7 MPa, which was relatively high value compared with the in vivo loading condition. Saikko [37] examined the effect of contact pressure on the polyethylene wear in his CTPOD device and reported artifactitious protuberance formation on the polyethylene worn surfaces with the contact pressure above 2 MPa. A similar protuberance formation was also observed in the current study and it was probably caused by applying high contact pressure to the specimens. Saikko and co-authors
655
had also reported the significant effects of other test conditions on the wear characteristics of UHMWPE evaluated their CTPOD device [38,39]. The basic mechanism of our multidirectional pin-on-plate wear tester is in common with that of CTPOD. Therefore, it is necessary to re-examine test conditions in future experiments to confirm the effects of protein and phospholipid concentrations on UHMWPE wear under physiological kinematics. 5. Conclusions Although there are several factors in the physiological environment of synovial joints affecting the wear behaviour of UHMWPE, the results of this study indicate that the macromolecules contained in synovial fluid, the protein and phospholipid molecules, have important roles in the in vivo UHMWPE wear mechanism. UHMWPE exhibited a small amount of wear in the nonphysiological macromolecule-free environment. However, the UHMWPE wear rate increased with increasing ambient protein concentrations. The phospholipid concentration also has significant effects on the polyethylene wear and increased the polyethylene wear rate in the solution containing physiological amounts of protein molecules. Acknowledgments The authors would like to thank Mr. S. Oda and Y. Fukumori, Kyushu University. All pin and plate specimens were supplied by Japan Medical Materials Corporation. This study was financially supported by the Grant-in-Aid for Scientific Research of Japan Society for the Promotion of Science. References [1] Willert HG, Semlitsch M. Reactions of the articular capsule to wear products of artificial joint prostheses. J Biomed Res 1977;11(2): 157–64. [2] Ingham E, Fisher J. Biological reactions to wear debris in total joint replacement. Proc Inst Mech Eng Part H 2000;214(1):21–37. [3] Bradley JS, Evans EJ. Carbon fibre reinforced ultra-high molecular weight polyethylene for medical application. Eng Med 1977;6(4): 123–4. [4] Schmidt RH. Osteolysis: new polymers and new solutions. Orthopedics 1994;17(9):817–8. [5] McKellop H, Shen FW, Lu B, Campbell P, Salovey R. Development of an extremely wear-resistant ultra high molecular weight polyethylene for total hip replacements. J Orthop Res 2003;17(2):157–67. [6] Muratoglu OK, Bragdon CR, O’Connor DO, Jasty M, Harris WH. A novel method of cross-linking ultra-high-molecular-weight polyethylene to improve wear, reduce oxidation, and retain mechanical properties. J Arthroplasty 2001;16(2):149–60. [7] Oonishi H, Kadoya Y, Masuda S. Gamma-irradiated cross-linked polyethylene in total hip replacements—analysis of retrieved sockets after long-term implantation. J Biomed Mater Res 2001;58(2): 167–71. [8] Moro T, Takatori Y, Ishihara K, Konno T, Takigawa Y, Matsushita T, et al. Surface grafting of artifi cial joints with a biocompatible polymer for preventing periprosthetic osteolysis. Nat Mater 2004; 3(11):829–36.
ARTICLE IN PRESS 656
Y. Sawae et al. / Tribology International 41 (2008) 648–656
[9] Eyerer P, Ke YC. Property changes of UHMW polyethylene hip cup endoprostheses during implantation. J Biomed Mater Res 1984;18(9): 1137–51. [10] Kurth M, Eyerer P, Ascherl R, Dittel K, Holz U. An evaluation of retrieved UHMWPE hip joint cups. J Biomater Appl 1988;3(1):22–51. [11] Costa L, Luda MP, Trossarelli L, Brach del Prever EM, Crova M, Gallinaro P. In vivo UHMWPE biodegradation of retrieved prosthesis. Biomaterials 1998;19(15):1371–85. [12] Ambrosio L, Carotenuto G, Marletta G, Nicolais L, Scandurra A. Wear effects in retrieved acetabular UHMW-PE cups. J Mater Sci Mater Med 1996;7(12):723–9. [13] Wang A, Essener A, Polineni VK, Stark C, Dumbleton JH. Lubrication and wear of ultra-high molecular weight polyethylene in total joint replacements. Tribol Int 1998;31(1-3):17–33. [14] Saikko V. Effect of lubricant protein concentration on the wear of ultra-high molecular weight polyethylene sliding against a CoCr counterface. J Tribol 2003;125:638–42. [15] Wang A, Essener A, Schmidig G. The effects of lubricant composition on in vitro wear testing of polymeric acetabular components. J Biomed Mater Res Part B Appl Biomater 2004;68B:45–52. [16] Sawae Y, Murakami T, Chen J. Effect of synovia constituents on friction and wear of ultra-high molecular weight polyethylene sliding against prosthetic joint materials. Wear 1998;216(2):213–9. [17] Saikko V, Ahlroos T. Wear simulation of UHMWPE for total hip replacement with a multidirectional motion pin-on-disk device: effects of counterface material, contact area, and lubricant. J Biomed Mater Res 2000;49(2):147–54. [18] Sawae Y, Murakami T, Doi S. Worn surface characteristics of ultrahigh molecular weight polyethylene lubricated with bovine serum. Thinning Films and Tribological Interface. Amsterdam: Elsevier; 2000. p. 347–356. [19] Heuberger MP, Widmer MR, Zobeley E, Glockshuber R, Spencer ND. Protein-mediated boundary lubrication in arthroplasty. Biomaterials 2005;26(10):1165–73. [20] Saikko V. A multidirectional motion pin-on-disk wear test method for prosthetic joint materials. J Biomed Mater Res 1998;41:58–64. [21] Hills BA. Oligolamellar lubrication of joints by surface active phospholipid. J Orthop Rheum 1989;16(1):82–91. [22] Hills BA. Boundary lubrication in vivo. Proc Inst Mech Eng Part H 2000;214(1):83–94. [23] Higaki H, Murakami T, Nakanishi Y, Miura H, Mawatari T, Iwamoto Y. The lubricating ability of biomembrane models with dipalmitoyl phosphatidylcholine and g-globulin. Proc Inst Mech Eng Part H 1998;212(5):337–46. [24] Mazzucco D, Scott R, Spector M. Composition of joint fluid in patients undergoing total knee replacement and revision arthroplasty: correlation with flow properties. Biomaterials 2004;25(18):4433–45. [25] James SP, Blazka S, Merrill EW, Jasty M, Lee KR, Bragdon GR, et al. Challenge to the concept that UHMWPE acetabular components oxidize in vivo. Biomaterials 1993;14(9):643–7.
[26] Costa L, Bracco P, Brach del Prever E, Luda MP, Trossarelli L. Analysis of products diffused into UHMWPE prosthetic components in vivo. Biomaterials 2001;22(4):307–15. [27] Greenbaum ES, Burroughs BB, Harris WH, Muratoglu OK. Effect of lipid adsorption on wear and compressive properties of unirradiated and highly crosslinked UHMWPE: an in vitro experimental model. Biomater 2004;25(18):4479–84. [28] Sawae Y, Murakami T, Sawano T. The influences of lipid and protein concentration on wear of ultra-high molecular weight polyethylene. Life cycle tribology. Amsterdam: Elsevier; 2005. p. 171–177. [29] Sawae Y, Murakami T. Role of synovia in wear mechanism of UHMWPE. Tribology in biomechanical systems, science and applications. Dusseldorf: Expert Verlag; 2001. p. 31–45. [30] Bell J, Tipper JL, Ingham E, Stone MH, Fisher J. The influence of phospholipid concentration in protein-containing lubricants on the wear of ultra-high molecular weight polyethylene in artificial hip joints. Proc Inst Mech Eng Part H 2001;215(2):259–63. [31] McKellop HA, Campbell P, Park SH, Schmalzried TP, Grigoris P, Amstutz HC, et al. The origin of submicron polyethylene wear debris in total hip arthroplasty. Clin Orthop Relat Res 1995;311: 3–20. [32] Wang A, Stark C, Dumbleton JH. Mechanistic and morphological origins of ultra-high molecular weight polyethylene wear debris in total joint replacement prostheses. Proc Inst Mech Eng Part H 1996;210(3):141–55. [33] Oonishi H, Tsuji E, Kim YY. Retrieved total hip prostheses. Part II wear behavior and structural changes. J Mater Sci Mater Med 1998;9:575–81. [34] Sawae Y, Murakami T. Wear mechanism of ultra-high molecular weight polyethylene in multidirectional sliding wear test. In: Proceedings of international tribology conference Nagasaki, 2000. JAST; 2001. p. 1521–7. [35] Bragdon CR, O’Connor DO, Lowenstein JD, Jasty M, Syniuta WD. The importance of multidirectional motion on the wear of polyethylene. Proc Inst Mech Eng Part H 1996;210(3):157–65. [36] Marrs H, Barton DC, Jones RA, Ward IM, Fisher J, Doyle C. Comparative wear under four different tribological conditions of acetylene enhanced cross-linked ultra high molecular weight polyethylene. Issue Series Title: J Mater Sci Mater Med 1999;10(6):333–42. [37] Saikko V. Effect of contact pressure on wear and friction of ultrahigh molecular weight polyethylene in multidirectional sliding. Proc Inst Mech Eng Part H 2006;220:723–31. [38] Saikko V, Calonius O, Keranen J. Effect of counterface roughness on the wear of conventional and crosslinked ultrahigh molecular weight polyethylene studied with a multi-directional motion pin-on-disk device. J Biomed Mater Res 2001;57:506–12. [39] Saikko V, Calonius O, Keranen J. Effect of slide track shape on the wear of ultra-high molecular weight polyethylene in a pin-on-disk wear simulation of total hip prosthesis. J Biomed Mater Res Part B 2004;69B:141–8.
Tribology International 41 (2008) 648–656 www.elsevier.com/locate/triboint
Influence of protein and lipid concentration of the test lubricant on the wear of ultra high molecular weight polyethylene Y. Sawaea,, A. Yamamotob, T. Murakamia a
Department of Intelligent Machinery and Systems, Faculty of Engineering, Kyushu University, 744 Motooka, Nishi-ku, Fukuoka 819-0395, Japan b Graduate School of Engineering, Kyushu University, 744 Motooka, Nishi-ku, Fukuoka 819-0395, Japan Received 9 February 2007; received in revised form 11 November 2007; accepted 17 November 2007 Available online 2 January 2008
Abstract To gain a better understanding of the ultra-high molecular weight polyethylene (UHMWPE) wear mechanism in the physiological environment, the effects of protein and lipid constituents of synovial fluid on the specific wear rate of UHMWPE were examined experimentally. The multidirectional sliding pin-on-plate wear tester was employed to simulate the simplified sliding condition of hip joint prostheses. Bovine serum g-globulin and synthetic L-a-DPPC were used as model protein and lipid constituents of synovia, respectively. Results of the wear test indicated that the UHMWPE wear rate primarily depended on the protein concentration of the test lubricant. Lipids acted as a boundary lubricant and reduced polyethylene wear in the low protein lubricants. However, the polyethylene wear rate increased with increasing lipid concentrations if the protein concentration was within the physiological level. Increased interactions between protein and lipid molecules and lipid diffusion to polyethylene surface might be responsible for the increased wear. r 2007 Elsevier Ltd. All rights reserved. Keywords: UHMWPE; Wear; Joint prosthesis; Protein; Phospholipid
1. Introduction Although various material combinations are used in the bearing surface of current joint prostheses, ultra-high molecular weight polyethylene (UHMWPE) is still the most popular bearing material for artificial joints. UHMWPE was first introduced in the bearing surface of hip joint prosthesis by Prof. Charnley since it seemed to have good tribological characteristics, low friction and low wear, and good biocompatibility as well. However, it has become evident that fine wear particles released from the UHMWPE component cause serious adverse reactions within the human body. They activate macrophages and cause necrosis of surrounding bone tissues. These histological changes finally induce the aseptic loosening of joint components [1,2]. Therefore, the wear characteristic of the UHMWPE is now recognized as determining factor of the
Corresponding author. Tel.: +81 92 802 3073; fax: +81 92 802 0001.
E-mail address: [email protected] (Y. Sawae). 0301-679X/$ - see front matter r 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.triboint.2007.11.010
durability and the long-term survivorship of artificial joint prostheses. Many attempts have been made to improve the wear property of UHMWPE [3–8]. The wear characteristics of newly developed materials should be examined in laboratory wear tests before clinical use. In in vitro wear estimations, the appropriate understanding of the in vivo wear mechanism is essential to reproduce the physiological wear behaviour and to obtain clinically reliable wear test results. However, the UHMWPE wear process in the human body is complicated, since joint prostheses are lubricated with synovia, which is a kind of body fluid secreted into the joint capsule and has a complicated composition including many kinds of electrolytes, radicals and biological macromolecules. These constituents are entrained into the contact zone during joint movement and might affect the friction and wear behaviour through chemical and mechanical interactions. Some ex vivo studies have suggested the considerable effects of the physiological environment on the material properties and subsequent wear behaviour of the implanted
ARTICLE IN PRESS Y. Sawae et al. / Tribology International 41 (2008) 648–656
UHMWPE. Eyerer and co-workers [9,10] indicated the accelerated oxidative degradation of the polyethylene articulating surface caused by the peroxide constituents of synovial fluid. The enhanced oxidation and subsequent mechanical property changes in the polyethylene surface were also demonstrated by Costa et al. [11] and Ambrosio et al. [12]. They described these adverse effects of the physiological environment as ‘‘biodegradation’’ and ‘‘biological corrosion’’, respectively. The influence of biological macromolecules contained in the synovial fluid has been examined by in vitro experimental methods. Specifically, the effects of serum proteins, the primal macromolecule constituent of synovial, on the wear behaviour of UHMWPE were examined by several research groups [13–19]. Wang et al. [13] focused on the protein concentration of test lubricants used in their simulator test and reported that the wear rate of the serum lubricated UHMWPE cup clearly increased by increasing the protein concentration of the lubricant. Similar results were also reported by Saikko [14] by using his circularly translating pin-on-disk (CTPOD) device [20]. Wang et al. [15] also indicated the effect of protein type and their relative concentration in the lubricant. Phospholipids are another macromolecule constituent of the synovial fluid, and its role as a boundary lubricant in the natural synovial joints has been demonstrated by Hills [21,22] and other researchers [23]. In contrast to proteins, there are a limited number of reports about the influence of phospholipid molecules on the tribological behaviour of prosthetic joint materials. However, Mazzucco et al. reported that the phospholipid concentration in synovial fluid obtained from joints implanted with joint prostheses is at the same level as that obtained from diseased joints [24], and several ex vivo analyses of retrieved polyethylene components indicate that the lipid constituents of synovia can diffuse into UHMWPE in vivo [25,26]. Greenbaum et al. [27] examined the effects of diffused lipid molecules on the mechanical properties and wear characteristics of UHMWPE experimentally. Their test results indicated that the lipid molecules that diffused into the bulk of the material had a significant plasticising effect on UHMWPE. Although the apparent effect of lipid molecules on the polyethylene wear behaviour could not be identified under their test conditions, the material property change in the bearing surface could alter the contact condition that might affect the material deformation in the real contact area and subsequent tribological conditions. Therefore, effects of lipid molecules on friction and wear should be postulated carefully. The final goal of this study is to understand the in vivo wear mechanism of UHMWPE and to identify the critical factors determining the wear behaviour of UHMWPE in a physiological environment. In the current study, we focused on biological macromolecules, specifically protein and phospholipid molecules, contained in synovia and experimentally examined their influence on the UHMWPE wear.
649
2. Materials and methods 2.1. Multidirectional pin-on-plate wear test The wear behaviour of UHMWPE sliding against medical grade anti-corrosive metal was evaluated by using a three-station multidirectional sliding pin-on-plate wear tester. This wear test apparatus was developed in our laboratory to simulate the polyethylene wear in hip joint by the simplified multidirectional sliding motion [28]. The basic principle of the multidirectional sliding in the present wear tester is identical to that of the CTPOD device developed by Saikko [20]. Fig. 1 is a schematic drawing of the multidirectional sliding wear tester used in this study. Three pin specimens were attached to loading bars and loaded individually by dead weights against a mated plate specimen. Each plate specimen was installed in a separate liquid bath and mounted on the X–Y stage which was moved by a motor and crank mechanism. A sliding motion arising between a pin and plate specimen in this apparatus is shown schematically in Fig. 2. In this case, the fixed pin specimen articulates upon the plate specimen surface along a circular path with a diameter of 30 mm. Then, the velocity vector is varied from v to v0 while the centre of the plate specimen moves from o to o0 . As a result, the direction of the sliding velocity vector changes continuously relative to the UHMWPE pin surface whereas the magnitude of the velocity vector remains constant at the any point on the pin surface. Cylindrical polyethylene pin specimens with a diameter of 6 mm were cut from GUR415 bar stock and were not
Fig. 1. Three-station multidirectional sliding pin-on-plate wear tester.
ARTICLE IN PRESS 650
Y. Sawae et al. / Tribology International 41 (2008) 648–656
Fig. 2. Schematic drawing of multidirectional sliding motion between pin and plate specimen.
sterilized prior to the wear tests. Cast Co–Cr–Mo alloy (ASTM-F75) was used as a plate specimen. The plate surface was polished by diamond slurry and the resultant surface roughness was measured by a stylus roughness meter. The average and the standard deviation of measured Ra values were 0.009 and 0.005 mm, respectively. Prior to the wear test, both pin and plate specimens were washed in an ultrasonic cleaner with a detergent (polyoxyethylene p-toctylphenylether) for 30 min. Then the specimens were rinsed in a stream of distilled water and washed in the ultrasonic cleaner again for 30 min with distilled water. Finally, the specimens were washed ultrasonically in ethanol for 15 min and dried in a vacuum desiccator. Wear tests were conducted at room temperature. A mean contact pressure of 7 MPa and a magnitude of the sliding velocity vector of 20 mm/s were applied between pin and plate specimens. Each test was run for a sliding distance of 10 km. The wear amount of the pin specimen was determined from the weight loss. At intervals of 5 km sliding, the pin specimen was removed from the test apparatus and cleaned by the same procedure described before. Then the specimen was dried in a vacuum desiccator for 2 h at 50 1C and weighed to an accuracy of 0.01 mg. A soak control pin was prepared to estimate the weight gain due to fluid uptake from the lubricant. Then, the specific wear rate, defined as the wear volume per unit load and unit sliding distance, was calculated from the weight loss of the UHMWPE pin specimens. After the wear test, morphological characteristics of the polyethylene worn surface were examined by optical microscopy. Furthermore, microscopic and nanoscopic structures of the worn surface were observed by atomic force microscopy (AFM) to examine the wear mechanism of UHMWPE. 2.2. Test lubricant Several phosphate buffered saline (PBS) solutions containing protein and lipid were prepared and used in
the wear test as lubricants. In this study, bovine serum g-globulin was chosen as a representative protein contained in synovia since it exhibited a more significant effect compared with albumin in our previous study [28,29]. Synthetic phospholipid (L-a-dipalmitoyl phosphatidylcholine, DPPC) was used as an alternative material for lipid constituents of synovial fluid. Typical preparation method of liposome was employed to disperse DPPC molecules in PBS as fine colloidal particles. In this method, an appropriate amount of DPPC powder was dissolved in chloroform. The DPPC solution was then poured into a flask and the solvent was subsequently removed by a rotary evaporator yielding a thin lipid film on the flask surface. The resultant lipid thin film was hydrated with PBS and finally ultrasonicated to disperse the DPPC vesicles homogeneously in PBS. Subsequently, g-globulin was dissolved in the DPPC dispersion. A 0.3 wt% sodium azide was also added to prevent bacterial growth. In this study, test lubricants with different compositions were prepared, and the effects of protein and lipid concentrations of the lubricant on UHMWPE wear were examined. The protein concentration was varied from a relatively small value of 0.1 wt% to physiological protein concentrations of synovial fluid, up to 2.0 wt% [23,24]. PBS solutions containing physiological amounts of protein molecules, 1.0 and 2.0 wt%, were defined as high protein lubricants, while the solutions with a protein concentration of 0.1, 0.2 and 0.4 wt% were categorized as low protein lubricants. On the other hand, the lipid concentrations of test lubricants used in this study were varied within the physiological range of synovial phospholipid concentration, 0.005, 0.01 and 0.02 wt% [23,24]. Diluted bovine serum (30 vol% fetal bovine serum+70 vol% distilled water) and PBS containing no macromolecules were used as control lubricants. The diluted bovine serum used in this study has a protein concentration of 1.1 wt% consisting of an albumin content of 0.7 wt% and a globulin content of 0.4 wt%. 3. Results Figs. 3–5 show the average weight loss of UHMWPE pin specimens as a function of the sliding distance. Fig. 3 is the comparison between the diluted bovine serum and the macromolecule-free PBS. The wear amount of UHMWPE pin specimens linearly increased with the sliding distance in the diluted bovine serum while the UHMWPE wear was almost negligible in the macromolecule-free PBS. Wear behaviours of UHMWPE pin specimens in the low protein lubricants with globulin concentration of 0.2 wt% were shown in Fig. 4. In this case, the wear amount increased almost linearly in lubricants with DPPC concentration of 0.005 and 0.01 wt%. However, polyethylene wear was suppressed after 5 km sliding if the DPPC concentration was increased to 0.02 wt%. The wear behaviour in high protein lubricants with globulin concentration of 1.0 wt%
ARTICLE IN PRESS Y. Sawae et al. / Tribology International 41 (2008) 648–656
651
Fig. 3. Mean wear amount of UHMWPE pin specimens lubricated with 30 vol% bovine serum solution and macromolecular-free PBS plotted against sliding distance. N ¼ 3 for the serum solution and N ¼ 1 for PBS.
Fig. 6. Comparison of UHMWPE specific wear rate between 30 vol% bovine serum solution and macromolecular-free PBS. Graph indicates mean value of pin specimens. Error bar means minimum and maximum value. N ¼ 3 for the serum solution and N ¼ 1 for PBS. Fig. 4. Mean wear amount of UHMWPE pin specimens lubricated with the low protein lubricant (g-globulin 0.2 wt%) plotted against sliding distance. N ¼ 3 for DPPC 0.005 and 0.01 wt% and N ¼ 2 for DPPC 0.02.
Fig. 5. Mean wear amount of UHMWPE pin specimens lubricated with the high protein lubricant (g-globulin 1.0 wt%) plotted against sliding distance. N ¼ 3 for all lubricants.
was compared in Fig. 5. In this case, the polyethylene wear amount increased linearly in all lubricants, and the wear amount increased with increasing phospholipid concentrations. The wear amount of UHMWPE pin specimens increased linearly during 10 km sliding and the transition from the initial wear to the steady state wear could not be identified in most cases. Therefore, all wear amount obtained during
Fig. 7. Effect of phopholipid concentration on specific wear rate of UHMWPE in low protein lubricants. Graph indicates mean value of pin specimens. Error bar means minimum and maximum value. N ¼ 3 except all g-globulin 0.1 wt% lubricants and g-globulin 0.2 wt% lubricant with DPPC 0.02 wt%, for which N ¼ 2.
10 km sliding was included in the calculation of the specific wear rate. The specific wear rates of UHMWPE in the 30 vol% serum solution and the macromolecule-free PBS were shown in Fig. 6. The average value of specific wear rate was found to be 3.47 107 mm3/Nm in diluted bovine serum. On the other hand, the wear rate in
ARTICLE IN PRESS 652
Y. Sawae et al. / Tribology International 41 (2008) 648–656
macromolecular-free PBS was 0.05 107 mm3/Nm, which is about one hundredth of that in the serum solution. The UHMWPE wear rates in low protein lubricants are compared in Fig. 7. Polyethylene wear rates were significantly small in low protein solutions compared with that in the serum solution. In lubricants containing only 0.1 wt% globulin, the average wear rates were 0.48 107, 0.66 107 and 0.18 107 mm3/Nm for the DPPC concentration of 0.005, 0.01 and 0.02 wt%, respectively. Then, the UHMWPE wear rate gradually increased to
Fig. 8. Effect of phopholipid concentration on specific wear rate of UHMWPE in high protein lubricants. Graph indicates mean value of pin specimens. Error bar means minimum and maximum value. N ¼ 3 for all lubricants.
1.06 107 and 1.89 107 mm3/Nm with increasing globulin concentration to 0.2 and 0.4 wt%, respectively, with the lowest DPPC concentration of 0.005 wt%. However, the wear rate in the lubricants containing 0.4 wt% globulin could be reduced to 0.78 107 and 0.52 107 mm3/Nm by increasing DPPC concentration to 0.01 and 0.02 wt%, respectively. UHMWPE wear rates in the high protein lubricants were shown in Fig. 8. The polyethylene wear rates in the high protein lubricants were apparently higher than those in the low protein lubricants, and the wear rate clearly increased with increasing lipid concentrations in this case. The average wear rate increased gradually from 2.30 107 to 3.03 107 and 3.52 107 mm3/Nm in the 1.0 wt% globulin solutions by increasing DPPC concentration from 0.005 to 0.01 and 0.02 wt%, respectively. Similar results were also obtained in the lubricants containing 2.0 wt% globulin. In this case, the average wear rate increased from 2.06 107 to 3.50 107 and 3.74 107 mm3/Nm by increasing the DPPC concentration. The average values of the UHMWPE wear rate are summarized and plotted against the protein concentration of test lubricants in Fig. 9. The wear rates in the diluted bovine serum and PBS were also plotted in the same figure. The UHMWPE wear rate primarily depended on the protein concentration of the lubricant and increased with increasing protein concentrations. However, the relationship between the polyethylene wear rate and the protein concentration of the test lubricant highly depended on the added phospholipid concentration. When the lubricant has a low phospholipid concentration of 0.005 wt%, the wear rate increased logarithmically with protein concentration and peaked at a protein concentration of 1.0 wt%. On the other hand, the increase in the polyethylene wear rate was limited within low protein regime if phospholipid concentration increased to 0.01 wt%. In this case, the polyethylene wear rate subsequently increased steeply by increasing protein concentration to 1.0 wt%, which is in a range of
Fig. 9. Average of UHMWPE wear rate plotted against protein concentration of lubricant.
ARTICLE IN PRESS Y. Sawae et al. / Tribology International 41 (2008) 648–656
653
Fig. 10. Comparison of optical micrographs from worn surface of UHMWPE pin specimens. (a) PBS, (b) g-globuline 1.0 wt%+DPPC 0.005 wt%, (c) g-globuline 1.0 wt%+DPPC 0.02 wt% and (d) g-globuline 2.0 wt%+DPPC 0.02 wt%.
physiological protein concentration of synovia, and became comparable to that in the diluted bovine serum. Optical micrographs from polyethylene worn surfaces are compared in Fig. 10. The initial surface of a pin specimen was covered with machinemarks, which formed during the specimen fabrication process, and many of them remained on the polyethylene worn surface in the macromolecular-free PBS (Fig. 10(a)). The visible machinemarks decreased as protein and phospholipid were added to PBS (Fig. 10(b)), and they were completely worn out if the globulin concentration and DPPC concentration of the lubricant were increased to 0.1 and 0.02 wt%, respectively (Fig. 10(c)). In this case, small protuberances could be found on the polyethylene worn surface. The polyethylene worn surface finally became very smooth in the lubricant containing 0.2 wt% globulin and 0.02 wt% DPPC. AFM images of the polyethylene worn surface are also compared in Fig. 11. Apparent differences in microscopic morphologies were observed between the macromolecular-free PBS and test lubricants containing protein and phospholipid; the polyethylene worn surfaces were covered with small asperities in test lubricants (Fig. 10(b–d)), although they were not observed on the worn surface in the macromolecular-free PBS (Fig. 10(a)). The size of asperities increased with increasing phospholipid concentration and fine fibrillated structures could be observed in the lubricant containing 2.0 wt% globulin and 0.02 wt% DPPC.
4. Discussion First, the polyethylene wear rate in macromolecule-free PBS was compared with the wear rate in the diluted bovine serum. The results imply the crucial effect of the macromolecules contained in bovine serum, since the wear amount of UHMWPE in PBS, which contains only several kinds of electrolytes, was almost negligible compared with that in the serum solution. This result is consistent with the results from the hip joint simulator test reported by Wang et al. [13]. In the current study, protein and phospholipid molecules were subsequently added to the PBS and their concentrations were changed to identify the influence of each constituent on the polyethylene wear behaviour. The specific wear rate of UHMWPE primarily depended on the protein concentration of the test lubricant. It gradually increased with increasing protein concentration of the lubricant. Similar results were also reported by Wang et al. [13] and Saikko [14]. They used pure water or distilled water as a control lubricant and the protein concentration of their test lubricant was adjusted by diluting bovine serum. On the other hand, the protein concentration of the test lubricant was regulated by adding serum proteins to PBS in this study. AFM observation revealed that the microscopic morphology of the polyethylene worn surface was significantly altered by protein molecules added to PBS. Fine asperities formed on the
ARTICLE IN PRESS 654
Y. Sawae et al. / Tribology International 41 (2008) 648–656
Fig. 11. Microscopic morphology of polyethylene worn surfaces observed by AFM. (a) PBS, (b) g-globuline 1.0 wt%+DPPC 0.005 wt%, (c) g-globuline 1.0 wt%+DPPC 0.02 wt% and (d) g-globuline 2.0 wt%+DPPC 0.02 wt%.
worn surface suggest the increased plastic deformation of the polyethylene surface in the lubricant containing protein molecules. Heuberger et al. [19] suggested that the protein molecules in lubricants could be denatured during a sliding test and unfolded proteins preferably adsorbed onto hydrophobic polyethylene surface. The adsorbed proteins might increase adhesion friction which enhanced microscopic plastic deformation of polyethylene surface and finally brought some changes in the polyethylene wear behaviour. The phospholipid concentration also had certain effects on the polyethylene wear rate in our experiments. However, the relationship between the phospholipid concentration of the lubricant and the wear rate of UHMWPE was somewhat complicated. The wear rate of UHMWPE was reduced by increasing the DPPC concentration of the test lubricant, if the protein concentration of the lubricant was lower than the physiological value for synovia, which is 20–40 mg/ml [13,14,23,24]. The polyethylene wear reduction in the low protein lubricant might suggest the potential of the phospholipid molecules as a boundary lubricant for
the prosthetic joint materials. The supplemented DPPC molecules might successfully form a boundary lubrication film on the material surfaces in the low protein lubricant and reduce polyethylene wear. The boundary lubrication effect of phospholipid for the artificial joint was also reported by Bell et al. [30], although the phospholipid concentration of their lubricants was higher than the physiological lipid concentration of synovial fluid. However, the boundary lubrication ability of the phospholipid molecules was suppressed by increasing the protein concentration to the physiological range, and inversely, the wear rate of UHMWPE increased by increasing the lipid concentration of the lubricant. The increased interaction between protein and phospholipid molecules is one of the possible reasons for the decrease in lipid boundary lubrication. Since lipid molecules can easily bind to proteins to form lipoproteins, the characteristics of the adsorbed boundary film may be altered by increased protein molecules. As a result, the adsorbed boundary film may lose its functional structure and lubrication abilities.
ARTICLE IN PRESS Y. Sawae et al. / Tribology International 41 (2008) 648–656
In addition to the protein–lipid interaction, there should be some interactions between phospholipid molecules and bearing surface materials. Several ex vivo analyses of retrieved polyethylene acetabular cups revealed that the lipid constituents of synovial fluid adsorbed on the polyethylene component and diffused into the bulk material in vivo [25,26], while evidence of the protein adsorption was observed only on the polyethylene surface [12,26]. Greenbaum et al. [27] examined the effect of diffused lipid molecules on the material properties of UHMWPE experimentally. They doped UHMWPE with cholestene and squalene and evaluate the changes in the compressive modulus and compressive strength of UHMWPE. Their experimental results clearly demonstrated the plasticizing effect of diffused lipid molecules since the compressive modulus and the compressive strength of UHMWPE significantly decreased after the lipid diffusion. These property changes might be responsible for the difference in the microscopic morphology of polyethylene worn surfaces observed in this study. AFM images indicated that the size and interval of small asperities, which formed on the polyethylene surface during sliding tests, became larger as the lipid concentration of the test lubricant increased. These results may suggest the plasticizing effect of phospholipid molecules added to the lubricant, which possibly alters the extent of microscopic plastic deformation in the polyethylene surfaces and consequently affect polyethylene wear behaviour. Furthermore, fine fibrils of the elongated polymer could be observed on the polyethylene worn surface only if the lubricant had high protein and phospholipid concentrations. As reported previously, a fine fibrillated microstructure is one of the common morphological characteristics observed in the load bearing area of retrieved polyethylene cups [31–33]. Wang et al. pointed out that the direction of the sliding motion in the laboratory wear test should be changed relative to the polyethylene surface during the experiment to reproduce the in vivo wear behaviour of UHMWPE [32]. The importance of the multidirectional sliding motion in the UHMWPE wear estimation was subsequently confirmed by other research groups [20,34–36]. Therefore, the multidirectional sliding pin-on-plate wear tester was employed to evaluate UHMWPE wear in this study. However, the worn surface morphologies observed by optical and atomic force microscopes were not necessarily in common with those found in retrieved polyethylene cups. In this study, wear tests were conducted with an average contact pressure of 7 MPa, which was relatively high value compared with the in vivo loading condition. Saikko [37] examined the effect of contact pressure on the polyethylene wear in his CTPOD device and reported artifactitious protuberance formation on the polyethylene worn surfaces with the contact pressure above 2 MPa. A similar protuberance formation was also observed in the current study and it was probably caused by applying high contact pressure to the specimens. Saikko and co-authors
655
had also reported the significant effects of other test conditions on the wear characteristics of UHMWPE evaluated their CTPOD device [38,39]. The basic mechanism of our multidirectional pin-on-plate wear tester is in common with that of CTPOD. Therefore, it is necessary to re-examine test conditions in future experiments to confirm the effects of protein and phospholipid concentrations on UHMWPE wear under physiological kinematics. 5. Conclusions Although there are several factors in the physiological environment of synovial joints affecting the wear behaviour of UHMWPE, the results of this study indicate that the macromolecules contained in synovial fluid, the protein and phospholipid molecules, have important roles in the in vivo UHMWPE wear mechanism. UHMWPE exhibited a small amount of wear in the nonphysiological macromolecule-free environment. However, the UHMWPE wear rate increased with increasing ambient protein concentrations. The phospholipid concentration also has significant effects on the polyethylene wear and increased the polyethylene wear rate in the solution containing physiological amounts of protein molecules. Acknowledgments The authors would like to thank Mr. S. Oda and Y. Fukumori, Kyushu University. All pin and plate specimens were supplied by Japan Medical Materials Corporation. This study was financially supported by the Grant-in-Aid for Scientific Research of Japan Society for the Promotion of Science. References [1] Willert HG, Semlitsch M. Reactions of the articular capsule to wear products of artificial joint prostheses. J Biomed Res 1977;11(2): 157–64. [2] Ingham E, Fisher J. Biological reactions to wear debris in total joint replacement. Proc Inst Mech Eng Part H 2000;214(1):21–37. [3] Bradley JS, Evans EJ. Carbon fibre reinforced ultra-high molecular weight polyethylene for medical application. Eng Med 1977;6(4): 123–4. [4] Schmidt RH. Osteolysis: new polymers and new solutions. Orthopedics 1994;17(9):817–8. [5] McKellop H, Shen FW, Lu B, Campbell P, Salovey R. Development of an extremely wear-resistant ultra high molecular weight polyethylene for total hip replacements. J Orthop Res 2003;17(2):157–67. [6] Muratoglu OK, Bragdon CR, O’Connor DO, Jasty M, Harris WH. A novel method of cross-linking ultra-high-molecular-weight polyethylene to improve wear, reduce oxidation, and retain mechanical properties. J Arthroplasty 2001;16(2):149–60. [7] Oonishi H, Kadoya Y, Masuda S. Gamma-irradiated cross-linked polyethylene in total hip replacements—analysis of retrieved sockets after long-term implantation. J Biomed Mater Res 2001;58(2): 167–71. [8] Moro T, Takatori Y, Ishihara K, Konno T, Takigawa Y, Matsushita T, et al. Surface grafting of artifi cial joints with a biocompatible polymer for preventing periprosthetic osteolysis. Nat Mater 2004; 3(11):829–36.
ARTICLE IN PRESS 656
Y. Sawae et al. / Tribology International 41 (2008) 648–656
[9] Eyerer P, Ke YC. Property changes of UHMW polyethylene hip cup endoprostheses during implantation. J Biomed Mater Res 1984;18(9): 1137–51. [10] Kurth M, Eyerer P, Ascherl R, Dittel K, Holz U. An evaluation of retrieved UHMWPE hip joint cups. J Biomater Appl 1988;3(1):22–51. [11] Costa L, Luda MP, Trossarelli L, Brach del Prever EM, Crova M, Gallinaro P. In vivo UHMWPE biodegradation of retrieved prosthesis. Biomaterials 1998;19(15):1371–85. [12] Ambrosio L, Carotenuto G, Marletta G, Nicolais L, Scandurra A. Wear effects in retrieved acetabular UHMW-PE cups. J Mater Sci Mater Med 1996;7(12):723–9. [13] Wang A, Essener A, Polineni VK, Stark C, Dumbleton JH. Lubrication and wear of ultra-high molecular weight polyethylene in total joint replacements. Tribol Int 1998;31(1-3):17–33. [14] Saikko V. Effect of lubricant protein concentration on the wear of ultra-high molecular weight polyethylene sliding against a CoCr counterface. J Tribol 2003;125:638–42. [15] Wang A, Essener A, Schmidig G. The effects of lubricant composition on in vitro wear testing of polymeric acetabular components. J Biomed Mater Res Part B Appl Biomater 2004;68B:45–52. [16] Sawae Y, Murakami T, Chen J. Effect of synovia constituents on friction and wear of ultra-high molecular weight polyethylene sliding against prosthetic joint materials. Wear 1998;216(2):213–9. [17] Saikko V, Ahlroos T. Wear simulation of UHMWPE for total hip replacement with a multidirectional motion pin-on-disk device: effects of counterface material, contact area, and lubricant. J Biomed Mater Res 2000;49(2):147–54. [18] Sawae Y, Murakami T, Doi S. Worn surface characteristics of ultrahigh molecular weight polyethylene lubricated with bovine serum. Thinning Films and Tribological Interface. Amsterdam: Elsevier; 2000. p. 347–356. [19] Heuberger MP, Widmer MR, Zobeley E, Glockshuber R, Spencer ND. Protein-mediated boundary lubrication in arthroplasty. Biomaterials 2005;26(10):1165–73. [20] Saikko V. A multidirectional motion pin-on-disk wear test method for prosthetic joint materials. J Biomed Mater Res 1998;41:58–64. [21] Hills BA. Oligolamellar lubrication of joints by surface active phospholipid. J Orthop Rheum 1989;16(1):82–91. [22] Hills BA. Boundary lubrication in vivo. Proc Inst Mech Eng Part H 2000;214(1):83–94. [23] Higaki H, Murakami T, Nakanishi Y, Miura H, Mawatari T, Iwamoto Y. The lubricating ability of biomembrane models with dipalmitoyl phosphatidylcholine and g-globulin. Proc Inst Mech Eng Part H 1998;212(5):337–46. [24] Mazzucco D, Scott R, Spector M. Composition of joint fluid in patients undergoing total knee replacement and revision arthroplasty: correlation with flow properties. Biomaterials 2004;25(18):4433–45. [25] James SP, Blazka S, Merrill EW, Jasty M, Lee KR, Bragdon GR, et al. Challenge to the concept that UHMWPE acetabular components oxidize in vivo. Biomaterials 1993;14(9):643–7.
[26] Costa L, Bracco P, Brach del Prever E, Luda MP, Trossarelli L. Analysis of products diffused into UHMWPE prosthetic components in vivo. Biomaterials 2001;22(4):307–15. [27] Greenbaum ES, Burroughs BB, Harris WH, Muratoglu OK. Effect of lipid adsorption on wear and compressive properties of unirradiated and highly crosslinked UHMWPE: an in vitro experimental model. Biomater 2004;25(18):4479–84. [28] Sawae Y, Murakami T, Sawano T. The influences of lipid and protein concentration on wear of ultra-high molecular weight polyethylene. Life cycle tribology. Amsterdam: Elsevier; 2005. p. 171–177. [29] Sawae Y, Murakami T. Role of synovia in wear mechanism of UHMWPE. Tribology in biomechanical systems, science and applications. Dusseldorf: Expert Verlag; 2001. p. 31–45. [30] Bell J, Tipper JL, Ingham E, Stone MH, Fisher J. The influence of phospholipid concentration in protein-containing lubricants on the wear of ultra-high molecular weight polyethylene in artificial hip joints. Proc Inst Mech Eng Part H 2001;215(2):259–63. [31] McKellop HA, Campbell P, Park SH, Schmalzried TP, Grigoris P, Amstutz HC, et al. The origin of submicron polyethylene wear debris in total hip arthroplasty. Clin Orthop Relat Res 1995;311: 3–20. [32] Wang A, Stark C, Dumbleton JH. Mechanistic and morphological origins of ultra-high molecular weight polyethylene wear debris in total joint replacement prostheses. Proc Inst Mech Eng Part H 1996;210(3):141–55. [33] Oonishi H, Tsuji E, Kim YY. Retrieved total hip prostheses. Part II wear behavior and structural changes. J Mater Sci Mater Med 1998;9:575–81. [34] Sawae Y, Murakami T. Wear mechanism of ultra-high molecular weight polyethylene in multidirectional sliding wear test. In: Proceedings of international tribology conference Nagasaki, 2000. JAST; 2001. p. 1521–7. [35] Bragdon CR, O’Connor DO, Lowenstein JD, Jasty M, Syniuta WD. The importance of multidirectional motion on the wear of polyethylene. Proc Inst Mech Eng Part H 1996;210(3):157–65. [36] Marrs H, Barton DC, Jones RA, Ward IM, Fisher J, Doyle C. Comparative wear under four different tribological conditions of acetylene enhanced cross-linked ultra high molecular weight polyethylene. Issue Series Title: J Mater Sci Mater Med 1999;10(6):333–42. [37] Saikko V. Effect of contact pressure on wear and friction of ultrahigh molecular weight polyethylene in multidirectional sliding. Proc Inst Mech Eng Part H 2006;220:723–31. [38] Saikko V, Calonius O, Keranen J. Effect of counterface roughness on the wear of conventional and crosslinked ultrahigh molecular weight polyethylene studied with a multi-directional motion pin-on-disk device. J Biomed Mater Res 2001;57:506–12. [39] Saikko V, Calonius O, Keranen J. Effect of slide track shape on the wear of ultra-high molecular weight polyethylene in a pin-on-disk wear simulation of total hip prosthesis. J Biomed Mater Res Part B 2004;69B:141–8.