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Tribology of UHMWPE tested against a stainless steel counterface in unidirectional sliding in presence of model synovial fluids: part 1
Wear 223 Ž1998. 13–21
Tribology of UHMWPE tested against a stainless steel counterface in unidirectional sliding in presence of model synovial fluids: part 1 Margam Chandrasekaran 1, Lee Yong Wei 2 , Krishna Kumar Venkateshwaran 3, Andrew William Batchelor ) , Nee Lam Loh 4 School of Mechanical and Production Engineering, Nanyang Technological UniÕersity, Singapore, 639798, Singapore Received 28 January 1998; revised 30 June 1998; accepted 17 August 1998
Abstract Friction and wear tests have been performed on ultra-high molecular weight polyethylene ŽUHMWPE. in presence of proteins, dry sliding conditions against a steel counterface disc and the results have been analyzed in detail. UHMWPE was found to exhibit lowest friction coefficient and wear rates when lubricated with bovine a globulin and with bovine albumin respectively. Post-test analysis of the proteins indicated denaturing, formation of reaction products of specimens and proteins. The predominant wear mechanisms found were adhesion, abrasion and fatigue. q 1998 Published by Elsevier Science S.A. All rights reserved. Keywords: Ultra-high molecular weight polyethylene; Tribology; Stainless steel counterface
1. Introduction Human joints form a naturally occurring tribological system that experiences lower friction and wear through effective lubrication by synovial fluids in body. Notwithstanding their excellent tribological properties, joint diseases due to various inflammation or accidents can impair the functionality of the joint resulting in a need for joint replacement w1x. Degenerative osteo-arthritis and rheumatoid arthritis are among such diseases, which often require the orthopaedic implant prosthesis as a cure. Various materials have been used so far as orthopaedic implants. The most important orthopaedic problem that needs addressing is kneerhip joint replacements where in combination to mechanical strength and chemical stability the material should be highly wear-resistant as relative sliding takes place at the joints. Most earlier designs of total hip joint replacements were done using metallic materials such as
1
Research EngineerrFellow. Associate Professor. 3 FYP Student. ) Corresponding author. Faculty of Engineering, Department of Applied Chemistry, Iwate University, Japan 4 Research Scholar. 2
Co–Cr. alloys or ceramic materials like TiO 2 , ZrO 2 and Al 2 O 3 owing to the distinct advantages of such materials like mechanical strength, chemical stability and wear resistance, etc. However, wear at the bearing surfaces in metallic materials and the lacuna of lower fracture toughness, ductility, loosening due to twisting at the hip and other problems in ceramics led to development of polymer-based materials w2–5x. High-density high molecular weight ŽHDHMW. polyethylene was selected for fabrication of acetabular cup w6x to overcome problems associated with wear of Co-Cr. alloy and most ceramic implant materials, as well as distortion and wear of PTFE. However, creep deformation, plastic distortion and high wear rates resulted with the use of HDHMWPE w7x. The other primary concern involving the usage of UHMWPE is the generation of wear debris, which has been found to induce adverse cellular reactions, bone re-sorption or osteolysis, resulting in joint loosening. This then necessitates undesirable revision operations w8x. Mechanical abrasion or attrition by environmental forces and physical stress results in biodegradation of the polymer w9,10x. Much work in biomedical engineering is aimed at minimizing polyethylene wear w11x. Before much progress can be done in this area, it is imperative to understand the tribological properties and wear mechanism of UHMWPE when lubricated with the principal compo-
0043-1648r98r$ - see front matter q 1998 Published by Elsevier Science S.A. All rights reserved. PII: S 0 0 4 3 - 1 6 4 8 Ž 9 8 . 0 0 2 9 4 - 4
M. Chandrasekaran et al.r Wear 223 (1998) 13–21
14
nents of synovial fluid. The ideal tribosystem of cartilage on cartilage have very low coefficients of friction of 0.003–0.02 in the presence of natural synovial fluid, has high load carrying capacity and a long life w12x. Natural synovial joints are composed of bones covered by a layer of compliant and porous articular cartilage, which acts as a bearing material, and the lubrication mechanism is considered as an adaptive multi-mode system w13x. The present work is aimed at evaluation of the behavior of UHMWPE slid against a 316 stainless steel disc counterface under lubrication with components of synovial fluid Žalbumin, a-globulin, g-globulin. thus simulating the actual combination of materials in acetabular cup replacements.
2. Experimental
Table 2 Experimental conditions Applied load Sliding distance Test duration
75 N 36.17 km 8h
materials used and the test conditions are tabulated in Tables 1 and 2. 3. Results The following paragraphs present the results obtained during tribological testing of UHMWPE in the presence of model proteins and post-test analysis and microscopy of the worn specimens. 3.1. Friction coefficient
A tri-pin-on-disc tribometer was used to test the friction and wear of UHMWPE w14x. To run the test, the pins are raised by pneumatic system to mate with a stainless steel counterface, which is driven by an electric motor. A load cell placed between the pin holder and pneumatic cylinder measures the normal force applied to the pins during the test. Friction force is measured with a full-bridge strain gauge positioned near the pin holder and seated on the platform. The wear depth was measured using a LVDT. Readings were recorded using a data logger. Nitrogen blanketing was provided to prevent from oxidation. After each test, the worn pin surface and the counterface were examined under scanning electron microscopy ŽSEM.. The surface roughness of the pin and counterface measured using a Taylor–Hobson, profilometer were 2.59 mm and 0.06 mm Ra before the start of test, respectively. For these experiments, pins of diameter 4.5 mm and disc 80 mm were employed. To detect the change in viscosity of the proteins used as lubricants in the wear test, a viscometer was used. X-ray photoelectron spectroscopy was performed on the lubricant samples collected before and after testing using electron spectroscopy chemical analysis ŽESCA.. The UHMWPE pins of molecular weight 4,500,000 was used in the current test and was fabricated by turning the samples to size in a CNC lathe. The
Table 1 Materials for experiments Pin ŽUHMWPE. Modulus of elasticity Diameter of pins Molecular weight
500 Nrmm2 4.0 mm 4,500,000
Disc Ž316 stainless steel. Modulus of elasticity Thickness Diameter of disc
194,000 Nrmm2 10.0 mm 80 mm
Fig. 1 shows the variation in friction coefficient with respect to time for test duration of 480 min. The behaviour in friction coefficient was similar for all proteins except bovine g globulin. The friction coefficient was found to be higher in case of dry sliding possibly due to direct contact of the surfaces without a separating film. The coefficient of friction for bovine g globulin showed a sudden increase after about 50 and 120 min during testing. At these two instances in the test, a stick-slip vibration occurred during the sliding. Also at these instances, the lubricating film of retained test fluid, agglomerated and formed a thick sludge which adhered to both the sliding surfaces. The coefficient of friction at the 50- and 120-min marks were 0.55 and 0.42, respectively, both much higher than that for dry sliding but the overall coefficient was lower than that of dry sliding. For bovine albumin, coefficient of friction fluctuated between 0.045 to 0.09 during the first 100 min, before stabilizing from 0.05 to 0.06. It was due possibly to a stable lubricating film formed between the counterface and pin. The average coefficient of friction of this test was slightly higher than bovine a globulin; this may due to the fact that albumin Ž1.30E0 Pa s. is more viscous than globulin Ž1.27E-03 and 1.21E-03 Pa s for a and g globulin, respectively.. Hence, it appears that the viscous drag of the protein layer was controlling the frictional force. For bovine a globulin, the coefficient of friction remained stable over the testing period. The low coefficient of friction for the test could be due to the viscosity of a globulin which possibly provided a hydrodynamic effect large enough to separate the asperities of the two surfaces but low enough to be easily sheared by the sliding surfaces. The viscosity of a globulin Ž1.27E-03 Pa s. was found to be lower than bovine albumin but higher than g globulin Ž1.21E-03 Pa s.. 3.2. Wear The wear rates calculated from the displacement of the LVDT are analyzed in the following paragraphs.
M. Chandrasekaran et al.r Wear 223 (1998) 13–21
15
Fig. 1. Graph of friction coefficient versus time under various sliding conditions.
Fig. 2 shows the variation in wear depth with respect to time for test duration of 480 min. The behaviour for each of the protein was different. The wear depth was a maximum for bovine g globulin. In the case of bovine g globulin spikes were observed near the 50-min mark similar to the friction behaviour. This corresponds to the earlier assumption that the stick-slip phenomenon observed at this time was a result of sludge formation. Negative wear was observed after 50 min, which may be due to a formation of a semi-solid lubricating film giving rise to a pseudo lift off. For dry sliding the wear depth increased for the first 20 min after which it started to stabilise in a similar manner to the behaviour of friction coefficient. The wear depth of bovine a globulin was higher than that obtained for albumin. The increased wear depth for both the globulins used is possible due to the corrosive action of the bovine a globulin. For bovine albumin, the wear depth increased up to 120 min after which it started to stabilize.
The formation of a stable lubricating film would have led to the reduction in wear. As albumin is more viscous Ž1.30E-03 Pa s. than globulin, the hydrodynamic lift of albumin is possibly higher than for globulin. Therefore, the lubricating film was able to provide a better separation between the asperities of the sliding surfaces and also maintaining the wear rate lower than that of globulin’s. 3.3. Analysis and microscopy of worn surface The worn specimens and counterface were examined using scanning electron microscopy ŽSEM.. Surfaces were sputtered with a gold coating to render them electrically conducting prior to the examination in the Cambridge scanning electron microscope. The lubricant was also analysed for the changes in viscosity before and after wear test. SEM observations of the counterface shown in Fig. 3a, revealed adhesion Žlumpy transfer layer on the steel identi-
Fig. 2. Graph of wear depth Žmm. versus time.
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M. Chandrasekaran et al.r Wear 223 (1998) 13–21
SEM images for bovine albumin counterface revealed the presence of a lumpy film comprising of bovine albumin and UHMWPE as shown in Fig. 4a. SEM images of the pin surface ŽFig. 4b. indicated a smooth surface in general and a layer peeled off from the surface of the pin at isolated points. At a much higher magnification ŽFig. 4c., the pin surface revealed the presence of striations and smaller debris. The striations are indicative that fatigue wear was present during the test. SEM images of the counterface did reveal pitting marks Fig. 5a. This corrosion is catalysed by heat and vibration from the sliding of the two surfaces. The presence of oxygen even with nitrogen blanketing would have aided in the corrosion process. Lumpy lubricating film was found to coat the wear tracks on the counterface shown in Fig. 5b. The lumpy film on the counterface resembled neither protein nor UHMWPE which possibly leads to the conclusion that a-globulin is slightly corrosive leading to forma-
Fig. 3. Ža. Lumpy transfer film on steel counterface Ždry sliding.. Žb. Ploughing marks and fatigue striations on pin surface. Žc. Loss of material by fatigue or adhesion Ždry sliding..
fied as a chunk of material on the steel. to be one of the predominant wear mechanisms during dry sliding. Ploughing marks and striations were observed on the pin surface. Fig. 3Žb,c. reveals that the predominant wear mechanisms during dry sliding were abrasive and fatigue wear. This can be identified by the fatigue striations on the pin surface with some score marks Ža score line near the top edge of the photograph.. It can be also seen that some flakes of UHMWPE had transferred on the steel counterface. This flaky transfer of UHMWPE is possibly due to fatigue wear of surface asperities on pin which got pressed to form thin flaky wear sheets and adhered on the counterface. Fig. 3a also shows a filament like structure which is due to rolling pressed wear sheets during sliding. Fig. 3b confirms the presence of fatigue wear and striations on surface of the pin. Fig. 3c, at a higher magnification, indicates removal of material by adhesion.
Fig. 4. Ža. SEM image of counterface lubricated with bovine albumin. Žb. SEM image of pin surface lubricated with bovine albumin showing a relatively smooth surface. Žc. SEM image of pin surface lubricated with bovine albumin showing fatigue striations and wear debris.
M. Chandrasekaran et al.r Wear 223 (1998) 13–21
Fig. 5. Ža. SEM image of counterface Žlubricated with a-globulin. showing pitting marks. Žb. SEM image of counterface Žlubricated with a-globulin. revealing lumpy lubricating film adhering to surface. Žc. SEM image of pin surface revealing fatigue wear striations and corrosive wear Žlubricated with a-globulin.. Žd. SEM image of the pin surface Žlubricated with a-globulin at double the original concentration 2.52 grl..
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SEM of bovine a globulin pin surface showed striations flaking and peeling of the UHMWPE near the turning marks Fig. 5c. The striations are indicative of fatigue wear. The flaking of the pin surface indicates the possibility of a reaction between a-globulin and the UHMWPE. To determine whether a-globulin was really corrosive, another test was conducted with double the concentration of a-globulin Ž2.52 grl.. Even the second test could not be performed for more than 1 h due to the damage to the pin surface as shown in Fig. 5d. SEM imaging of the counterface revealed lubricating films adhered along the wear track. The films had wavy surfaces with rolled particles that could eventually become wear debris Fig. 6a. It is possible that these films are a result of corrosive wear of UHMWPE and the resulting film adhered to the counterface due to polarity. At various places along the film, large entanglements of wear particles or protein, such as those in Fig. 6b, may have impeded sliding of the two surfaces, resulting in increased coefficient of friction. When the lubricating film was removed from the counterface, deep pits were observed along the wear track Fig. 6c, indicating that g-globulin is corrosive during sliding as observed from the brownish reaction compounds on the disk and pin surfaces. The pits on the counterface are filled up with lubricating film. It is possible that these pits formed early in the sliding test. The stick-slip phenomenon observed was due possibly to the plastic distortion of UHMWPE with sliding and subsequent wear of material being caught in these pits, resulting in massive surface deformation and loss of material. When the pins were observed under the SEM, Ploughing marks, such as those in Fig. 6d, could be very clearly seen on the pin surface. This shows that abrasive wear mechanism was predominant to the overall wear of the UHMWPE pins. The deep scratches observed suggests that a three-body abrasive wear mechanism could have prevailed during the sliding test. The lubricating film, which is sludge like, could trap wear particles and drag it along the pin surface, much like sandpaper with fixed ceramic grits. If weakening of the pin surface occurred, then the amount of material lost by abrasion would be much greater. It was inferred from the SEM image that delamination was also operative, indicating that pin had undergone subsurface deformation. The formation of pits and cracks ŽFig. 6e. evidences corrosive wear at the edge of pin. Hence, abrasion, corrosion and fatigue cause the high wear rates observed for the pin. The finding that wear of UHMWPE is caused by adhesion, abrasion, fatigue and corrosion is consistent with the earlier literature w17x. 3.4. Changes in Õiscosity
tion of such transfer film resulting from corrosion. Earlier works on the Ti-6A1-4V implants also indicated wear accelerated corrosive behavior of such implants w15,16x
The lubricant was analysed for the changes in viscosity before and after wear test. Viscosity test showed that
M. Chandrasekaran et al.r Wear 223 (1998) 13–21
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Fig. 6. Ža. SEM image of lubricating film on counterface Žlubricated with g-globulin.. Žb. SEM image showing entanglement of wear particles along wear track of counterface Žlubricated with g-globulin.. Žc. SEM image of counterface showing pitting marks Žlubricated with g-globulin.. Žd. SEM image of pin surface Žlubricated with g-globulin. showing numerous Ploughing marks. Že. SEM image of pin surface showing delamination of the UHMWPE.
bovine albumin, a-globulin are pseudoplastic and gglobulin have rheopectic behaviour. Their physical changes are tabulated in Table 3.
The results tabulated are the overall shift observed in the respective peaks after testing with the corresponding protein. A shift of 0.2 eV indicated a change in the
Table 3 Viscosity and visual changes in proteins after testing Synovial fluid
Viscosity changes
Post-test color
Possible reasons
Albumin
Dynamic viscosity was 1.30E-03 Pa.s and 1.15E-03 Pa.s before and after the wear test Dynamic viscosity was 1.20E-03 Pa.s and 1.14E-03 Pa.s before and after the wear test Dynamic viscosity was 1.21E-03 Pa.s and 1.27E-03 Pa.s before and after the wear test
Cloudy and yellowish
Denaturing
Slightly cloudy
Denaturing
Cloudy fluid containing suspension of brown particles
Chemisorption leading to corrosion and transfer of debris
Bovine a-globulin
Bovine g-globulin
M. Chandrasekaran et al.r Wear 223 (1998) 13–21
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Table 4 ESCA Results of the components tested Elements Protein
Peak shift for Carbon
Peak shift for Nitrogen
Peak shift for Oxygen
Peak shift for Sulphur
Peak shift for Chlorine
Iron
Bovine Albumin a-Globulin g-Globulin
y0.36 q0.14 y0.43
q1.43 y0.69 y1.36
y0.49 q0.55 q0.25
y0.69 -
-
712.98
electronic states of the element representing some adverse reactions with the counterfacerpin surface material. ESCA results reveal that reactions were present during the tests Table 4. The corrosive nature of g-globulin is also evident from the iron peak, which appears after testing, indicating that some iron wear particle is being carried away by the protein. These results also support the theory of degradation of protein during the friction wear test in presence of the steel and UHMWPE resulting in changes in the viscosity of the protein as well. The presence of iron peak and
peak shifts also suggests corrosive wear of UHMWPE and steel.
4. Discussion The UHMWPE was found to have uniform coefficient of friction and lower wear rates in presence of albumin and globulin except for g-globulin. The sudden increase and lowering of is due possibly to the formation of an insolu-
Fig. 7. Ža. Shearing. Žb. Degradation and corrosive reaction. Žc. Formation of wear debris and denaturing. Žd. Transfer film formation. Že. Removal of transfer layer.
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M. Chandrasekaran et al.r Wear 223 (1998) 13–21
ble sludge and breaking of sludge. This leads to fresh intake of globulin resulting in formation of thin film between the surfaces. The SEM pictures of worn specimen tested in the presence of g globulin also indicated that the pins had undergone a massive plastic deformation w11x. The agglomeration and corrosiveness of g-globulin outweigh the positive effects of viscosity on wear. The agglomeration of g-globulin is due possibly to the denaturing of the protein w11x. This may be due to the lower surface free energy influencing the electrical charge at the surface which have been found to affect the adhesion and configuration of proteins resulting in denaturing of proteins w18x. Explanted UHMWPE acetabular cup has been reported to be stained brown, probably due to inflammation, at high wear regions w19x. However, no other phenomenon could be observed. The brownish stain on the agglomerated protein could be a result of a reaction between UHMWPE and g-globulin. Although the coefficient of friction for g-globulin was lower than for dry sliding, the wear rate for g-globulin was higher. Such a trend was observed when UHMWPE was slid with 30% hydrogen peroxide w14x. The proteins was observed to have undergone denaturation in general during sliding as indicated by the pre- and post-test viscosity analysis showing a decreasing trend.
Ž1. Adhesion, fatigue, corrosive and abrasive wear were observed to be predominant mechanisms of wear present in the tests. Ž2. UHMWPE undergoes plastic distortion during sliding in presence of globulin. Ž3. Viscosity tests showed that Bovine albumin, aglobulin exhibited pseudoplastic and g-globulin rheopectic behaviour. The rheopectic behaviour is typical of synovial fluids. Ž4. Albumin, a-globulin, g-globulin were found to be corrosive to steel and UHMWPE. The reaction of these proteins with UHMWPE are likely to be catalysed by heat, cyclic vibration and shearing action resulting in denaturing of protein chains and corrosion is initiated by polarized denatured protein.
4.1. Model of eÕents
References
The wear mechanisms of UHMWPE with steel contacts have been summarized in a model of events shown in Fig. 7Ža–e.. The globulin molecules entrapped in the contact is deformed and sheared by the normal pressure combined with the sliding action and become denatured loosing the equilibrium structure releasing free radicals and resulting in increased reactivity. Relative sliding also induces fatigue cracks and abrasive score marks on the sliding surfaces. The denatured molecule of globulin reacts with the nascent steel surface and polymer ŽUHMWPE. surface by seeping through the fatigue cracks further weakening the surface molecular chains in the polymer and propagating a corrosion reaction in the steel surface. Thus, the crack propagates due to the combined chemical and mechanical action on polymer surface to generate wear debris. The generated wear debris may have lesser surface free energy and the higher surface area to volume than the bulk material thus catalyzing corrosive reactions. The reaction product of UHMWPE and globulin transfers to the counterface forming a transfer film. This transfer film is removed by further shearing. This cycle tends to repeat resulting in high wear rates of UHMWPE.
w1x J.H. Dumbleton, Tribology of Natural and Artificial Joints, Tribology Series 3, Elsevier, Amsterdam, 1981. w2x H. McKellop, B. Lu, P. Benya, Friction lubrication and wear of cobalt-chromium, alumina and Zirconia hip prostheses compared on a joint simulator, 38th Ann. Meet., Orthopaedic Research Society, February 17–20, 1992, Washington, DC. w3x H. Buetler, M. Lehmann, G. Stahli, Wear behaviour of medical engineering materials, Wear 33 Ž1975. 337. w4x H. Mckellop, I. Clarke, K. Markolf, H. Amsutz, Friction and wear properties of polymer, metal, and ceramic prosthetic joint materials evaluated on a multichannel screening device, Journal of Biomedical Materials Research 16 Ž1982. 177. w5x D.F. Williams, Review–tissue biomaterial interactions, Journal of Materials Science 22 Ž1987. 3421. w6x J. Charnley, The long term results of low friction arthoplasty of the hip performed as a primary invention, Journal of Bone Joint Surgery Br. 54B Ž1972. 61. w7x I. Duff-Barclay, D.T. Spillman, Total human hip joint prostheses—A laboratory study of friction and wear, Proc. Inst. Mechanical Eng. Part 3J, 181, 1966–1967, p. 90. w8x T. Wright, Trends in Research and Treatment of Joint Diseases, Springer, Japan 1992, p. 116. w9x Polymeric Biomaterials, Severian Dumitrium Publ. ŽEd.., Marcel Dekker, 1994, New York. w10x G.M. Ferguson, Biodegradation of Synthetic Polymers, Chemistry in Australia, January–February, 1991, p. 10. w11x G. William, Artificial Hip Joints - A Challenge For Ideas Concerning Tribology and Designing. Material, Wissenschaft and Werkstofftechnik, Vol. 27, Issue 4, VCH Publishers, 1996, FL. w12x H. Higaki, T. Murakami, The Boundary lubricating ability of proteins in natural synovial joints, Proc. of the International Tribology conference, Yokohama, Japan, Oct. 29–Nov. 2, 1995, Publ. Japanese Society of Tribologists, p. 1987. w13x T. Murakami, N. Ohtsuki, H. Higaki, The adaptive multimode
5. Conclusions The following conclusions can be made from the test results.
Acknowledgements The authors would like to gratefully acknowledge the support of the School of Mechanical and Production Engineering, Nanyang Technological University.
M. Chandrasekaran et al.r Wear 223 (1998) 13–21 lubrication in biotribological systems, Proc. of the International Tribology conference, Yokohama, Oct. 29–Nov. 2, 1995 Japan, Publ. Japanese Society of Tribologists, p. 1981. w14x A.W. Batchelor, B.P. Tan, Effect of an Oxidising agent on the friction and wear of Nylon 6 against a steel counterface, Proc. of the International Tribology conference, in: G.W. Staichowiak ŽEd.., Austrib 94, Perth, 5–8, Dec. 94, University of Western Australia, p. 175. w15x R.A. Buchanan, E.D. Rigney Jr., J.M. Williams, Wear accelerated corrosion of Ti-6Al-4V and nitrogen -ion implanted Ti-6Al-4V: mechanisms and influence of fixed-stress magnitude, Journal of Biomedical Materials Research 21 Ž1987. 367.
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w16x P. Eyerer, M. Kurth, H.A. Mckelleup, T. Mittlmeir, Characterization of UHMWPE Hip cups run on joint simulators, Journal of Biomedical Materials Research 21 Ž1987. 275. w17x R.A. Poggie, A.K. Mishra, J.A. Davidson, Three-body abrasive wear behaviour of orthopaedic implant bearing surfaces from Titanium debris, Journal of Materials Science: Materials in Medicine 5 Ž1994. 387. w18x Blackshear, The Chemistry of Biosurfaces, in: M.L. Hair ŽEd.., Vol. 2, Dekker, New York, 1972, p. 523. w19x L. Ambrosio, G. Cartenuto, G. Marletta, L. Nicolais, A. Scandurra, Wear effects in retrieved acetabular UHMWPE cups, Journal of Materials Science: Materials in Medicine 7 Ž1996. 723.
Tribology of UHMWPE tested against a stainless steel counterface in unidirectional sliding in presence of model synovial fluids: part 1 Margam Chandrasekaran 1, Lee Yong Wei 2 , Krishna Kumar Venkateshwaran 3, Andrew William Batchelor ) , Nee Lam Loh 4 School of Mechanical and Production Engineering, Nanyang Technological UniÕersity, Singapore, 639798, Singapore Received 28 January 1998; revised 30 June 1998; accepted 17 August 1998
Abstract Friction and wear tests have been performed on ultra-high molecular weight polyethylene ŽUHMWPE. in presence of proteins, dry sliding conditions against a steel counterface disc and the results have been analyzed in detail. UHMWPE was found to exhibit lowest friction coefficient and wear rates when lubricated with bovine a globulin and with bovine albumin respectively. Post-test analysis of the proteins indicated denaturing, formation of reaction products of specimens and proteins. The predominant wear mechanisms found were adhesion, abrasion and fatigue. q 1998 Published by Elsevier Science S.A. All rights reserved. Keywords: Ultra-high molecular weight polyethylene; Tribology; Stainless steel counterface
1. Introduction Human joints form a naturally occurring tribological system that experiences lower friction and wear through effective lubrication by synovial fluids in body. Notwithstanding their excellent tribological properties, joint diseases due to various inflammation or accidents can impair the functionality of the joint resulting in a need for joint replacement w1x. Degenerative osteo-arthritis and rheumatoid arthritis are among such diseases, which often require the orthopaedic implant prosthesis as a cure. Various materials have been used so far as orthopaedic implants. The most important orthopaedic problem that needs addressing is kneerhip joint replacements where in combination to mechanical strength and chemical stability the material should be highly wear-resistant as relative sliding takes place at the joints. Most earlier designs of total hip joint replacements were done using metallic materials such as
1
Research EngineerrFellow. Associate Professor. 3 FYP Student. ) Corresponding author. Faculty of Engineering, Department of Applied Chemistry, Iwate University, Japan 4 Research Scholar. 2
Co–Cr. alloys or ceramic materials like TiO 2 , ZrO 2 and Al 2 O 3 owing to the distinct advantages of such materials like mechanical strength, chemical stability and wear resistance, etc. However, wear at the bearing surfaces in metallic materials and the lacuna of lower fracture toughness, ductility, loosening due to twisting at the hip and other problems in ceramics led to development of polymer-based materials w2–5x. High-density high molecular weight ŽHDHMW. polyethylene was selected for fabrication of acetabular cup w6x to overcome problems associated with wear of Co-Cr. alloy and most ceramic implant materials, as well as distortion and wear of PTFE. However, creep deformation, plastic distortion and high wear rates resulted with the use of HDHMWPE w7x. The other primary concern involving the usage of UHMWPE is the generation of wear debris, which has been found to induce adverse cellular reactions, bone re-sorption or osteolysis, resulting in joint loosening. This then necessitates undesirable revision operations w8x. Mechanical abrasion or attrition by environmental forces and physical stress results in biodegradation of the polymer w9,10x. Much work in biomedical engineering is aimed at minimizing polyethylene wear w11x. Before much progress can be done in this area, it is imperative to understand the tribological properties and wear mechanism of UHMWPE when lubricated with the principal compo-
0043-1648r98r$ - see front matter q 1998 Published by Elsevier Science S.A. All rights reserved. PII: S 0 0 4 3 - 1 6 4 8 Ž 9 8 . 0 0 2 9 4 - 4
M. Chandrasekaran et al.r Wear 223 (1998) 13–21
14
nents of synovial fluid. The ideal tribosystem of cartilage on cartilage have very low coefficients of friction of 0.003–0.02 in the presence of natural synovial fluid, has high load carrying capacity and a long life w12x. Natural synovial joints are composed of bones covered by a layer of compliant and porous articular cartilage, which acts as a bearing material, and the lubrication mechanism is considered as an adaptive multi-mode system w13x. The present work is aimed at evaluation of the behavior of UHMWPE slid against a 316 stainless steel disc counterface under lubrication with components of synovial fluid Žalbumin, a-globulin, g-globulin. thus simulating the actual combination of materials in acetabular cup replacements.
2. Experimental
Table 2 Experimental conditions Applied load Sliding distance Test duration
75 N 36.17 km 8h
materials used and the test conditions are tabulated in Tables 1 and 2. 3. Results The following paragraphs present the results obtained during tribological testing of UHMWPE in the presence of model proteins and post-test analysis and microscopy of the worn specimens. 3.1. Friction coefficient
A tri-pin-on-disc tribometer was used to test the friction and wear of UHMWPE w14x. To run the test, the pins are raised by pneumatic system to mate with a stainless steel counterface, which is driven by an electric motor. A load cell placed between the pin holder and pneumatic cylinder measures the normal force applied to the pins during the test. Friction force is measured with a full-bridge strain gauge positioned near the pin holder and seated on the platform. The wear depth was measured using a LVDT. Readings were recorded using a data logger. Nitrogen blanketing was provided to prevent from oxidation. After each test, the worn pin surface and the counterface were examined under scanning electron microscopy ŽSEM.. The surface roughness of the pin and counterface measured using a Taylor–Hobson, profilometer were 2.59 mm and 0.06 mm Ra before the start of test, respectively. For these experiments, pins of diameter 4.5 mm and disc 80 mm were employed. To detect the change in viscosity of the proteins used as lubricants in the wear test, a viscometer was used. X-ray photoelectron spectroscopy was performed on the lubricant samples collected before and after testing using electron spectroscopy chemical analysis ŽESCA.. The UHMWPE pins of molecular weight 4,500,000 was used in the current test and was fabricated by turning the samples to size in a CNC lathe. The
Table 1 Materials for experiments Pin ŽUHMWPE. Modulus of elasticity Diameter of pins Molecular weight
500 Nrmm2 4.0 mm 4,500,000
Disc Ž316 stainless steel. Modulus of elasticity Thickness Diameter of disc
194,000 Nrmm2 10.0 mm 80 mm
Fig. 1 shows the variation in friction coefficient with respect to time for test duration of 480 min. The behaviour in friction coefficient was similar for all proteins except bovine g globulin. The friction coefficient was found to be higher in case of dry sliding possibly due to direct contact of the surfaces without a separating film. The coefficient of friction for bovine g globulin showed a sudden increase after about 50 and 120 min during testing. At these two instances in the test, a stick-slip vibration occurred during the sliding. Also at these instances, the lubricating film of retained test fluid, agglomerated and formed a thick sludge which adhered to both the sliding surfaces. The coefficient of friction at the 50- and 120-min marks were 0.55 and 0.42, respectively, both much higher than that for dry sliding but the overall coefficient was lower than that of dry sliding. For bovine albumin, coefficient of friction fluctuated between 0.045 to 0.09 during the first 100 min, before stabilizing from 0.05 to 0.06. It was due possibly to a stable lubricating film formed between the counterface and pin. The average coefficient of friction of this test was slightly higher than bovine a globulin; this may due to the fact that albumin Ž1.30E0 Pa s. is more viscous than globulin Ž1.27E-03 and 1.21E-03 Pa s for a and g globulin, respectively.. Hence, it appears that the viscous drag of the protein layer was controlling the frictional force. For bovine a globulin, the coefficient of friction remained stable over the testing period. The low coefficient of friction for the test could be due to the viscosity of a globulin which possibly provided a hydrodynamic effect large enough to separate the asperities of the two surfaces but low enough to be easily sheared by the sliding surfaces. The viscosity of a globulin Ž1.27E-03 Pa s. was found to be lower than bovine albumin but higher than g globulin Ž1.21E-03 Pa s.. 3.2. Wear The wear rates calculated from the displacement of the LVDT are analyzed in the following paragraphs.
M. Chandrasekaran et al.r Wear 223 (1998) 13–21
15
Fig. 1. Graph of friction coefficient versus time under various sliding conditions.
Fig. 2 shows the variation in wear depth with respect to time for test duration of 480 min. The behaviour for each of the protein was different. The wear depth was a maximum for bovine g globulin. In the case of bovine g globulin spikes were observed near the 50-min mark similar to the friction behaviour. This corresponds to the earlier assumption that the stick-slip phenomenon observed at this time was a result of sludge formation. Negative wear was observed after 50 min, which may be due to a formation of a semi-solid lubricating film giving rise to a pseudo lift off. For dry sliding the wear depth increased for the first 20 min after which it started to stabilise in a similar manner to the behaviour of friction coefficient. The wear depth of bovine a globulin was higher than that obtained for albumin. The increased wear depth for both the globulins used is possible due to the corrosive action of the bovine a globulin. For bovine albumin, the wear depth increased up to 120 min after which it started to stabilize.
The formation of a stable lubricating film would have led to the reduction in wear. As albumin is more viscous Ž1.30E-03 Pa s. than globulin, the hydrodynamic lift of albumin is possibly higher than for globulin. Therefore, the lubricating film was able to provide a better separation between the asperities of the sliding surfaces and also maintaining the wear rate lower than that of globulin’s. 3.3. Analysis and microscopy of worn surface The worn specimens and counterface were examined using scanning electron microscopy ŽSEM.. Surfaces were sputtered with a gold coating to render them electrically conducting prior to the examination in the Cambridge scanning electron microscope. The lubricant was also analysed for the changes in viscosity before and after wear test. SEM observations of the counterface shown in Fig. 3a, revealed adhesion Žlumpy transfer layer on the steel identi-
Fig. 2. Graph of wear depth Žmm. versus time.
16
M. Chandrasekaran et al.r Wear 223 (1998) 13–21
SEM images for bovine albumin counterface revealed the presence of a lumpy film comprising of bovine albumin and UHMWPE as shown in Fig. 4a. SEM images of the pin surface ŽFig. 4b. indicated a smooth surface in general and a layer peeled off from the surface of the pin at isolated points. At a much higher magnification ŽFig. 4c., the pin surface revealed the presence of striations and smaller debris. The striations are indicative that fatigue wear was present during the test. SEM images of the counterface did reveal pitting marks Fig. 5a. This corrosion is catalysed by heat and vibration from the sliding of the two surfaces. The presence of oxygen even with nitrogen blanketing would have aided in the corrosion process. Lumpy lubricating film was found to coat the wear tracks on the counterface shown in Fig. 5b. The lumpy film on the counterface resembled neither protein nor UHMWPE which possibly leads to the conclusion that a-globulin is slightly corrosive leading to forma-
Fig. 3. Ža. Lumpy transfer film on steel counterface Ždry sliding.. Žb. Ploughing marks and fatigue striations on pin surface. Žc. Loss of material by fatigue or adhesion Ždry sliding..
fied as a chunk of material on the steel. to be one of the predominant wear mechanisms during dry sliding. Ploughing marks and striations were observed on the pin surface. Fig. 3Žb,c. reveals that the predominant wear mechanisms during dry sliding were abrasive and fatigue wear. This can be identified by the fatigue striations on the pin surface with some score marks Ža score line near the top edge of the photograph.. It can be also seen that some flakes of UHMWPE had transferred on the steel counterface. This flaky transfer of UHMWPE is possibly due to fatigue wear of surface asperities on pin which got pressed to form thin flaky wear sheets and adhered on the counterface. Fig. 3a also shows a filament like structure which is due to rolling pressed wear sheets during sliding. Fig. 3b confirms the presence of fatigue wear and striations on surface of the pin. Fig. 3c, at a higher magnification, indicates removal of material by adhesion.
Fig. 4. Ža. SEM image of counterface lubricated with bovine albumin. Žb. SEM image of pin surface lubricated with bovine albumin showing a relatively smooth surface. Žc. SEM image of pin surface lubricated with bovine albumin showing fatigue striations and wear debris.
M. Chandrasekaran et al.r Wear 223 (1998) 13–21
Fig. 5. Ža. SEM image of counterface Žlubricated with a-globulin. showing pitting marks. Žb. SEM image of counterface Žlubricated with a-globulin. revealing lumpy lubricating film adhering to surface. Žc. SEM image of pin surface revealing fatigue wear striations and corrosive wear Žlubricated with a-globulin.. Žd. SEM image of the pin surface Žlubricated with a-globulin at double the original concentration 2.52 grl..
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SEM of bovine a globulin pin surface showed striations flaking and peeling of the UHMWPE near the turning marks Fig. 5c. The striations are indicative of fatigue wear. The flaking of the pin surface indicates the possibility of a reaction between a-globulin and the UHMWPE. To determine whether a-globulin was really corrosive, another test was conducted with double the concentration of a-globulin Ž2.52 grl.. Even the second test could not be performed for more than 1 h due to the damage to the pin surface as shown in Fig. 5d. SEM imaging of the counterface revealed lubricating films adhered along the wear track. The films had wavy surfaces with rolled particles that could eventually become wear debris Fig. 6a. It is possible that these films are a result of corrosive wear of UHMWPE and the resulting film adhered to the counterface due to polarity. At various places along the film, large entanglements of wear particles or protein, such as those in Fig. 6b, may have impeded sliding of the two surfaces, resulting in increased coefficient of friction. When the lubricating film was removed from the counterface, deep pits were observed along the wear track Fig. 6c, indicating that g-globulin is corrosive during sliding as observed from the brownish reaction compounds on the disk and pin surfaces. The pits on the counterface are filled up with lubricating film. It is possible that these pits formed early in the sliding test. The stick-slip phenomenon observed was due possibly to the plastic distortion of UHMWPE with sliding and subsequent wear of material being caught in these pits, resulting in massive surface deformation and loss of material. When the pins were observed under the SEM, Ploughing marks, such as those in Fig. 6d, could be very clearly seen on the pin surface. This shows that abrasive wear mechanism was predominant to the overall wear of the UHMWPE pins. The deep scratches observed suggests that a three-body abrasive wear mechanism could have prevailed during the sliding test. The lubricating film, which is sludge like, could trap wear particles and drag it along the pin surface, much like sandpaper with fixed ceramic grits. If weakening of the pin surface occurred, then the amount of material lost by abrasion would be much greater. It was inferred from the SEM image that delamination was also operative, indicating that pin had undergone subsurface deformation. The formation of pits and cracks ŽFig. 6e. evidences corrosive wear at the edge of pin. Hence, abrasion, corrosion and fatigue cause the high wear rates observed for the pin. The finding that wear of UHMWPE is caused by adhesion, abrasion, fatigue and corrosion is consistent with the earlier literature w17x. 3.4. Changes in Õiscosity
tion of such transfer film resulting from corrosion. Earlier works on the Ti-6A1-4V implants also indicated wear accelerated corrosive behavior of such implants w15,16x
The lubricant was analysed for the changes in viscosity before and after wear test. Viscosity test showed that
M. Chandrasekaran et al.r Wear 223 (1998) 13–21
18
Fig. 6. Ža. SEM image of lubricating film on counterface Žlubricated with g-globulin.. Žb. SEM image showing entanglement of wear particles along wear track of counterface Žlubricated with g-globulin.. Žc. SEM image of counterface showing pitting marks Žlubricated with g-globulin.. Žd. SEM image of pin surface Žlubricated with g-globulin. showing numerous Ploughing marks. Že. SEM image of pin surface showing delamination of the UHMWPE.
bovine albumin, a-globulin are pseudoplastic and gglobulin have rheopectic behaviour. Their physical changes are tabulated in Table 3.
The results tabulated are the overall shift observed in the respective peaks after testing with the corresponding protein. A shift of 0.2 eV indicated a change in the
Table 3 Viscosity and visual changes in proteins after testing Synovial fluid
Viscosity changes
Post-test color
Possible reasons
Albumin
Dynamic viscosity was 1.30E-03 Pa.s and 1.15E-03 Pa.s before and after the wear test Dynamic viscosity was 1.20E-03 Pa.s and 1.14E-03 Pa.s before and after the wear test Dynamic viscosity was 1.21E-03 Pa.s and 1.27E-03 Pa.s before and after the wear test
Cloudy and yellowish
Denaturing
Slightly cloudy
Denaturing
Cloudy fluid containing suspension of brown particles
Chemisorption leading to corrosion and transfer of debris
Bovine a-globulin
Bovine g-globulin
M. Chandrasekaran et al.r Wear 223 (1998) 13–21
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Table 4 ESCA Results of the components tested Elements Protein
Peak shift for Carbon
Peak shift for Nitrogen
Peak shift for Oxygen
Peak shift for Sulphur
Peak shift for Chlorine
Iron
Bovine Albumin a-Globulin g-Globulin
y0.36 q0.14 y0.43
q1.43 y0.69 y1.36
y0.49 q0.55 q0.25
y0.69 -
-
712.98
electronic states of the element representing some adverse reactions with the counterfacerpin surface material. ESCA results reveal that reactions were present during the tests Table 4. The corrosive nature of g-globulin is also evident from the iron peak, which appears after testing, indicating that some iron wear particle is being carried away by the protein. These results also support the theory of degradation of protein during the friction wear test in presence of the steel and UHMWPE resulting in changes in the viscosity of the protein as well. The presence of iron peak and
peak shifts also suggests corrosive wear of UHMWPE and steel.
4. Discussion The UHMWPE was found to have uniform coefficient of friction and lower wear rates in presence of albumin and globulin except for g-globulin. The sudden increase and lowering of is due possibly to the formation of an insolu-
Fig. 7. Ža. Shearing. Žb. Degradation and corrosive reaction. Žc. Formation of wear debris and denaturing. Žd. Transfer film formation. Že. Removal of transfer layer.
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M. Chandrasekaran et al.r Wear 223 (1998) 13–21
ble sludge and breaking of sludge. This leads to fresh intake of globulin resulting in formation of thin film between the surfaces. The SEM pictures of worn specimen tested in the presence of g globulin also indicated that the pins had undergone a massive plastic deformation w11x. The agglomeration and corrosiveness of g-globulin outweigh the positive effects of viscosity on wear. The agglomeration of g-globulin is due possibly to the denaturing of the protein w11x. This may be due to the lower surface free energy influencing the electrical charge at the surface which have been found to affect the adhesion and configuration of proteins resulting in denaturing of proteins w18x. Explanted UHMWPE acetabular cup has been reported to be stained brown, probably due to inflammation, at high wear regions w19x. However, no other phenomenon could be observed. The brownish stain on the agglomerated protein could be a result of a reaction between UHMWPE and g-globulin. Although the coefficient of friction for g-globulin was lower than for dry sliding, the wear rate for g-globulin was higher. Such a trend was observed when UHMWPE was slid with 30% hydrogen peroxide w14x. The proteins was observed to have undergone denaturation in general during sliding as indicated by the pre- and post-test viscosity analysis showing a decreasing trend.
Ž1. Adhesion, fatigue, corrosive and abrasive wear were observed to be predominant mechanisms of wear present in the tests. Ž2. UHMWPE undergoes plastic distortion during sliding in presence of globulin. Ž3. Viscosity tests showed that Bovine albumin, aglobulin exhibited pseudoplastic and g-globulin rheopectic behaviour. The rheopectic behaviour is typical of synovial fluids. Ž4. Albumin, a-globulin, g-globulin were found to be corrosive to steel and UHMWPE. The reaction of these proteins with UHMWPE are likely to be catalysed by heat, cyclic vibration and shearing action resulting in denaturing of protein chains and corrosion is initiated by polarized denatured protein.
4.1. Model of eÕents
References
The wear mechanisms of UHMWPE with steel contacts have been summarized in a model of events shown in Fig. 7Ža–e.. The globulin molecules entrapped in the contact is deformed and sheared by the normal pressure combined with the sliding action and become denatured loosing the equilibrium structure releasing free radicals and resulting in increased reactivity. Relative sliding also induces fatigue cracks and abrasive score marks on the sliding surfaces. The denatured molecule of globulin reacts with the nascent steel surface and polymer ŽUHMWPE. surface by seeping through the fatigue cracks further weakening the surface molecular chains in the polymer and propagating a corrosion reaction in the steel surface. Thus, the crack propagates due to the combined chemical and mechanical action on polymer surface to generate wear debris. The generated wear debris may have lesser surface free energy and the higher surface area to volume than the bulk material thus catalyzing corrosive reactions. The reaction product of UHMWPE and globulin transfers to the counterface forming a transfer film. This transfer film is removed by further shearing. This cycle tends to repeat resulting in high wear rates of UHMWPE.
w1x J.H. Dumbleton, Tribology of Natural and Artificial Joints, Tribology Series 3, Elsevier, Amsterdam, 1981. w2x H. McKellop, B. Lu, P. Benya, Friction lubrication and wear of cobalt-chromium, alumina and Zirconia hip prostheses compared on a joint simulator, 38th Ann. Meet., Orthopaedic Research Society, February 17–20, 1992, Washington, DC. w3x H. Buetler, M. Lehmann, G. Stahli, Wear behaviour of medical engineering materials, Wear 33 Ž1975. 337. w4x H. Mckellop, I. Clarke, K. Markolf, H. Amsutz, Friction and wear properties of polymer, metal, and ceramic prosthetic joint materials evaluated on a multichannel screening device, Journal of Biomedical Materials Research 16 Ž1982. 177. w5x D.F. Williams, Review–tissue biomaterial interactions, Journal of Materials Science 22 Ž1987. 3421. w6x J. Charnley, The long term results of low friction arthoplasty of the hip performed as a primary invention, Journal of Bone Joint Surgery Br. 54B Ž1972. 61. w7x I. Duff-Barclay, D.T. Spillman, Total human hip joint prostheses—A laboratory study of friction and wear, Proc. Inst. Mechanical Eng. Part 3J, 181, 1966–1967, p. 90. w8x T. Wright, Trends in Research and Treatment of Joint Diseases, Springer, Japan 1992, p. 116. w9x Polymeric Biomaterials, Severian Dumitrium Publ. ŽEd.., Marcel Dekker, 1994, New York. w10x G.M. Ferguson, Biodegradation of Synthetic Polymers, Chemistry in Australia, January–February, 1991, p. 10. w11x G. William, Artificial Hip Joints - A Challenge For Ideas Concerning Tribology and Designing. Material, Wissenschaft and Werkstofftechnik, Vol. 27, Issue 4, VCH Publishers, 1996, FL. w12x H. Higaki, T. Murakami, The Boundary lubricating ability of proteins in natural synovial joints, Proc. of the International Tribology conference, Yokohama, Japan, Oct. 29–Nov. 2, 1995, Publ. Japanese Society of Tribologists, p. 1987. w13x T. Murakami, N. Ohtsuki, H. Higaki, The adaptive multimode
5. Conclusions The following conclusions can be made from the test results.
Acknowledgements The authors would like to gratefully acknowledge the support of the School of Mechanical and Production Engineering, Nanyang Technological University.
M. Chandrasekaran et al.r Wear 223 (1998) 13–21 lubrication in biotribological systems, Proc. of the International Tribology conference, Yokohama, Oct. 29–Nov. 2, 1995 Japan, Publ. Japanese Society of Tribologists, p. 1981. w14x A.W. Batchelor, B.P. Tan, Effect of an Oxidising agent on the friction and wear of Nylon 6 against a steel counterface, Proc. of the International Tribology conference, in: G.W. Staichowiak ŽEd.., Austrib 94, Perth, 5–8, Dec. 94, University of Western Australia, p. 175. w15x R.A. Buchanan, E.D. Rigney Jr., J.M. Williams, Wear accelerated corrosion of Ti-6Al-4V and nitrogen -ion implanted Ti-6Al-4V: mechanisms and influence of fixed-stress magnitude, Journal of Biomedical Materials Research 21 Ž1987. 367.
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w16x P. Eyerer, M. Kurth, H.A. Mckelleup, T. Mittlmeir, Characterization of UHMWPE Hip cups run on joint simulators, Journal of Biomedical Materials Research 21 Ž1987. 275. w17x R.A. Poggie, A.K. Mishra, J.A. Davidson, Three-body abrasive wear behaviour of orthopaedic implant bearing surfaces from Titanium debris, Journal of Materials Science: Materials in Medicine 5 Ž1994. 387. w18x Blackshear, The Chemistry of Biosurfaces, in: M.L. Hair ŽEd.., Vol. 2, Dekker, New York, 1972, p. 523. w19x L. Ambrosio, G. Cartenuto, G. Marletta, L. Nicolais, A. Scandurra, Wear effects in retrieved acetabular UHMWPE cups, Journal of Materials Science: Materials in Medicine 7 Ž1996. 723.