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Surface characterization of metal-on-metal hip implants tested in a hip simulator
Wear 225–229 Ž1999. 708–715
Surface characterization of metal-on-metal hip implants tested in a hip simulator A. Wang a
a,)
, S. Yue a , J.D. Bobyn b, F.W. Chan b, J.B. Medley
c
Metallurgical Engineering Department, McGill UniÕersity, MH Wong Building, 3610 UniÕersity St., Montreal, Quebec, H3A 2B2, Canada b Jo Miller Orthopaedic Research Laboratory, Montreal General Hospital and McGill UniÕersity, Montreal, Quebec, Canada c Mechanical Engineering Department, UniÕersity of Waterloo, Waterloo, Ontario, Canada
Abstract The purpose of this study was to characterize metallurgical and tribological events occurring at the articulating surfaces of metal–metal implants tested in a hip simulator in order to gain understanding of the wear characteristics of Co–Cr–Mo alloys. The surfaces of 12 implant heads, made of either cast, low carbon wrought, or high carbon wrought Co–Cr–Mo material, were examined using scanning electron and atomic force microscopes. Three of the implants were examined prior to simulator testing, three after three million cycles of testing, and six after six million cycles of testing. Initially the carbides in the cast and high carbon wrought components were proud of the surface. With testing, in the high carbon wrought components, the carbides were worn below the matrix surface and were also a source for micropits. In the cast components, some of the carbides remained proud of the surface, while others were worn below the matrix surface with increased test cycles. Some of the carbides in the cast alloy experienced partial or full pull-out, resulting in micropits. All three alloys showed evidence of matrix wear through a process resembling delamination in which layers of material appeared to be removed. q 1999 Elsevier Science S.A. All rights reserved. Keywords: Metal–metal hip implants; Surface characterization; Hip simulator
1. Introduction Metal–metal bearing surfaces in artificial hip replacement are increasingly gaining acceptance because of their excellent wear resistance w1x. Several North-American implant manufacturers have developed metal–metal prostheses for clinical trials w1,2x, while in Europe, over 60,000 metal–metal hip bearings have been implanted in the past decade w3–5x. Of the metals that are commonly used as biomaterials, cobalt–chromium–molybdenum ŽCo–Cr– Mo. alloys are the preferred materials for self-bearing applications w6,7x, and these alloys are classified by the American Society for Testing and Materials ŽASTM.. While the original metal–metal hip implants from three decades ago were typically made of cast Co–Cr–Mo alloy w6x, the modern generation implants are made of three different Co–Cr–Mo alloys. Each alloy possesses a different microstructure, and thus might experience different wear mechanisms and a different wear resistance. At present, the mechanisms of wear and the factors governing )
Corresponding author. Tel.: q1-514-398-4755 ext. 09511; fax: q1514-398-4492; e-mail: [email protected]
wear resistance of each alloy are not well understood. The purpose of the present study is to characterize metallurgical and tribological events that occur in regions of direct surface contact during hip simulator testing of metal–metal implants. However, the present study is confined to examining carbide morphology and behavior, as well as matrix wear mechanisms. Abrasion and, in particular, third-body abrasion is examined in another publication by Wang et al. w8x. Surface characterization will help in the understanding of the wear mechanisms of metal–metal hip implants and guide alloy selection. 2. Materials and methods The implants under investigation were manufactured from the following Co–Cr–Mo alloys: cast ŽASTM F7592., low carbon ŽLC. wrought ŽASTM F1537-94. or high carbon ŽHC. wrought ŽASTM F1537-94.. The ASTM composition standards for these alloys are given in Table 1. The typical carbon content of the LC wrought alloys is 0.05 wt.% and that of the cast and HC wrought is 0.2 wt.%. All three alloys have a large proportion of Cr, which is necessary in order to create a passive oxide film that
0043-1648r99r$ - see front matter q 1999 Elsevier Science S.A. All rights reserved. PII: S 0 0 4 3 - 1 6 4 8 Ž 9 8 . 0 0 3 8 4 - 6
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Table 1 ASTM composition standards for surgical implant applications Alloy
Composition Žwt.%. Cr
Mo C
Fe
Ni
Si
Mn N
Co
Cast F75-92 26–30 5–7 F 0.35 F 0.75 F1 F1 F1 F 0.25 Bal. Wrought 27–30 5–7 F 0.35 F 0.75 F1 F1 F1 0 Bal. F1537-94
contributes substantially to corrosion resistance w9x. The alloys also have 5–7 wt.% Mo for solid solution strengthening w10x. Carbon is added at levels of less than 0.35 wt.%, also for solid solution strengthening, but, more importantly, for the formation of carbides, which further strengthen the material and possibly improve wear resistance. The carbides present in the alloys are usually in the form of M 23 C 6 , where M is some combination of Co, Cr and Mo w11x. The microstructures of these alloys ŽFigs. 1 and 2. were obtained by sectioning three heads, the surfaces of which were not examined. These sections were ground and polished to a final polish of 0.05 mm alumina suspension. The polished surfaces were electroetched with 60% nitric acid in water kept at room temperature. The cast alloy was etched for a total of 12 s, the HC wrought for 18 s and the LC wrought for 20 s, all at 5 V. Prior to SEM examination, the microstructures were sputter coated with gold in order to render them conductive. In order to form the implants, the various alloys were ground and polished into spherical heads of 28 mm diameter and hemispherical acetabular cups, which, when mated with the heads, had a diametrical clearance of approximately 40–100 mm. For the present study, the articulating surfaces of 12 heads were examined. Three of the heads Žone of each alloy. were examined as manufactured Ži.e., without any hip simulator testing.. Three heads Žone of each alloy. had been tested in a model EW08 MMED hip simulator ŽMATCO, La Canada, CA. for three million cycles w12,13x, and six heads Žthree of the cast and three of the HC wrought. had been tested in the hip simulator for six million cycles. The hip simulator subjected the articulating surfaces to a biaxial rocking motion with a peak load of 2.1 kN applied in a Paul loading cycle at 1.13 Hz, approximating the resultant normal force during walking w14x. All tests were run in bovine serum containing antibacterial and antifungal agents w12,15x. The cycle motion of each wear station simulated about 468 of flexion-extension and involved multidirectional motion at the articulating surfaces as described by Medley et al. w16x. Testing methods and conditions are described in detail by Chan et al. w15x. Information on the specimens is summarized in Table 2, along with the mass loss resulting from simulator testing. The articulating surfaces of the heads were examined using a JEOL 840 scanning electron microscope ŽSEM. at
Fig. 1. Typical microstructure of the cast alloy obtained with SEM in secondary electron mode. Ža. At low power; Žb. region from the window in Ža.; Žc. region from the window in Žb..
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Fig. 2. Typical microstructures obtained with SEM in secondary electron mode. Ža. HC wrought alloy; Žb. LC wrought alloy.
accelerating voltages of 5 or 10 kV. The SEM was used for topographical and compositional imaging, as well as for qualitative elemental analysis using a NORAN I2 energy dispersive spectrometer ŽEDS. that was equipped with an ultra thin window for detection of low atomic number elements. The surfaces were prepared for examination by cleaning with acetone andror replica tape. They were not sputter coated with gold or carbon because they were to be subjected to further wear testing in the hip simulator. As a complementary analytical technique, a Digital Instruments Dimension 3100 atomic force microscope ŽAFM. was also used to analyze the heads. This enabled better resolution and quantitative topography to be performed.
3. Results All twelve heads displayed third body abrasive scratches as reported and discussed in detail by Wang et al. w8x. The Table 2 The mass loss experienced by each implant that was examined Alloy
Cycles of articulation Žmillion.
Mass loss Žmg.
Cast LC wrought HC wrought Cast LC wrought HC wrought Cast Cast Cast HC wrought HC wrought HC wrought
0 0 0 3 3 3 6 6 6 6 6 6
0 0 0 1.3 8.0 1.2 5.8 5.1 6.5 5.1 8.5 5.8
present paper, however, concentrates on carbide morphology and behavior, as well as matrix wear mechanisms Žaside from abrasive scratches.. The cast microstructure ŽFig. 1. was the coarsest of the three alloys, having grains with diameters of several hundreds of micrometers. Carbides outlined the grain boundaries and were also found within the grains. Those within the grains were either in chains or isolated. The carbide size ranged from about 1 to 10 mm in diameter. When wrought, the grains were reduced in diameter to 10 mm or less. The HC wrought alloy ŽFig. 2a. had carbides which ranged in diameter from about 1 to 5 mm and were found in abundance along the grain boundaries as well as within the grains. Unlike the cast alloy, the grain boundaries of the HC wrought alloy were not outlined by the carbides. Most of the carbides in the LC wrought alloy ŽFig. 2b. were less than 0.5 mm in diameter and were found at the grain boundaries as well as within the grains. Some carbides, about 1 mm in diameter, were also present and were noted only at the grain boundaries. When examining the surfaces of the implant heads using SEM accelerating voltages of 5 and 10 kV, the carbides in the cast and the HC wrought were apparent, while those in the LC wrought heads were not. Although the carbides in the LC wrought alloy could be seen in the microstructure ŽFig. 2b., those on the implant heads could not because the implants were not sputter-coated with gold and thus the required resolution could not be achieved. When subjected to EDS, the carbide regions were found to have carbon, chromium and molybdenum at levels higher than in the matrix, thus confirming the visual identification. 3.1. Prior to simulator testing Prior to testing, the SEM examination seemed to suggest that the detectable carbides in the as-manufactured
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Fig. 3. AFM images illustrating carbides on components prior to simulator testing. The darkest regions are deepest, lightest regions are most proud. Ža. Cast alloy, apex region. Note the proud nature of the carbide chain; Žb. HC wrought alloy, apex region. Note the proud isolated carbides and the surface scratches from the manufacturing process. Images were obtained in tapping mode and were flattened to remove curvature effect.
cast and HC wrought components were flush with the surface. However, AFM examination showed these carbides to be standing proud ŽFig. 3.. Cross-sectional analyses of the carbides showed them to be protruding several tens of nanometers on the cast component, and less than 10 nm on the HC wrought component. The carbides in the LC wrought component were not seen with the AFM. Unlike the SEM, resolution achieved with the AFM is sufficient to detect the small carbides in the LC wrought alloy. However, since they were not detected the carbides were probably flush with the surface.
bides were always darker because of a back-scattering electron effect w17x. This effect is lessened when the sample is coated with a heavy element, such as gold. That is the reason why the carbides in the sputter-coated microstructures appeared light ŽFigs. 1 and 2., while those in the uncoated implants appeared dark Že.g., Fig. 5.. The carbide behavior in the LC wrought head could not be determined because of insufficient resolution. At this stage, components of each alloy displayed matrix wear through a process that could be described as
3.2. After three million cycles of testing After three million cycles in the hip simulator, the cast implant experienced a total mass loss of 1.3 mg, the HC wrought implant 1.2 mg and the LC wrought implant 8.0 mg ŽTable 2.. The wear occurred in a zone at the apex of the implant head ŽFig. 4.. In the wear zone, all of the carbides in the HC wrought head and most of the carbides found in chains in the cast component were no longer protruding from the surface, but were present below the surface of the matrix. This indicated that the carbides had been worn away at a faster rate than the surrounding matrix, as illustrated for the HC wrought head ŽFig. 5.. It should be noted that on the cast and HC wrought heads, the carbides always appeared darker than the matrix during secondary electron imaging with the SEM, regardless of whether they were above, flush, or below the matrix. Despite the fact that secondary electron imaging primarily reveals topographical features, it is believed that the car-
Fig. 4. Illustration of hip implant components which were tested in the simulator and the apical wear zone on the head used for the microscopic analyses.
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Fig. 5. SEM micrograph of the apex region from a HC wrought component tested to three million cycles. Numerous carbides just below the surface are visible as well as an irregular boundary suggestive of matrix delamination. Image was obtained in secondary electron mode.
delamination. This type of wear appeared as a discontinuous matrix, displaying a fragmented upper layer that rested on a nonfragmented lower layer ŽFig. 5.. 3.3. After six million cycles of testing After six million cycles, the three HC wrought implants experienced a total mass loss of 5.1, 8.5, and 5.8 mg,
Fig. 7. SEM micrograph of the apex of a cast component tested to six million cycles. The pits within the carbide chain possess smooth edges whereas the pits at isolated carbide sites possess noticeably rougher edges. This may be related to carbide composition and mechanism of pit formation. Image was obtained in secondary electron mode.
respectively ŽTable 2.. The carbides in the HC wrought heads remained below the surface at the wear zone, similar to that which was observed after three million cycles. Carbide behavior in the LC wrought components could not be determined at this stage, as no LC wrought heads were
Fig. 6. Images illustrating a carbide chain on a cast component tested to six million cycles. The carbide labeled U serves to orient the chain on both images. Ža. SEM micrograph obtained in secondary electron mode. Subsurface as well as protruding carbides appearing dark; Žb. AFM image of the same chain in Ža. taken in tapping mode and flattened to remove curvature effect. The carbides within the chain are all subsurface and some are pitted. The light, isolated carbides outside the chain are proud of the surface.
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Fig. 8. Ža. AFM image of a large pit on a HC wrought component tested to six million cycles. Image was obtained in tapping mode and was flattened to remove curvature effect. Žb. Cross-sectional analysis of the pit indicating a width of 4.4 mm and a depth of 113 nm.
available for examination after six million cycles. Three cast implants were available for examination after six million cycles. It was observed that these three implants had different microstructures with different carbide distributions. Two of the implants had substantially more carbide chains than the third, which had mainly isolated carbides. The two implants with many chains experienced a weight loss of 5.8 and 5.1 mg, respectively, and the implant with mainly isolated carbides experienced a similar weight loss of 6.5 mg ŽTable 2.. At this stage, the carbides found in chains were not only present slightly below the matrix surface Žas was seen after three million cycles., but some were fractured and experienced partial pull-out, resulting in pits ŽFig. 6.. In the two implants that had multiple carbide chains, the isolated carbides rarely experienced pull-out. The implant that had mainly isolated carbides showed what appeared to be full pull-out of most of the isolated carbides ŽFig. 7.. However, the carbides that were present in chains on this implant displayed the kind of pitting seen on the other two implants ŽFig. 7.. The pits resulting from isolated carbide pull-out had considerably rougher edges than the pits resulting from the partial carbide pull-out in the carbide chains. After six million cycles of testing, pits were found on the HC wrought heads. The pits were found at the apex over an area of about 1–4 mm2 ŽFig. 8.. The diameters of these pits were of the same order of magnitude as the carbides Ž1–5 mm., but penetrated only to a depth of about 100–140 nm. The HC wrought implant that experienced the most weight loss Ž8.5 mg. had substantially more pits than did the other two HC wrought heads, some as large as
10 mm in diameter. The smaller pits were associated with carbides, suggesting that the pits originated at carbides. Since the depth of the pits was an order of magnitude smaller than the carbides, it is likely that carbides still existed at the bottom of the pits. As was observed after three million cycles of testing, the matrix still wore by delamination after six million cycles. The delaminating matrix was once again of two layers. In the HC wrought component, the upper, fragmented layer possessed carbides that were below the matrix surface. The carbides present in the nonfragmented, lower layer were flush with the surface of this layer. This
Fig. 9. SEM micrograph illustrating a zone of delamination at the periphery of the apical wear zone on a HC wrought component tested to six million cycles. Image was obtained in secondary electron mode.
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Fig. 10. SEM micrograph illustrating upper and lower delamination layers at the periphery of the apical wear zone on a cast component tested to six million cycles. Image was obtained in secondary electron mode.
carbide behavior is depicted in Fig. 9. AFM examination of the top layer revealed that the depth to which the carbides were below the top layer was similar to the thickness of this layer. The depth and thickness were found to be approximately 30–40 nm, respectively. In the cast component, the carbides were also believed to be below the surface of the upper layer, and flush with the surface of the lower layer ŽFig. 10.. On both the cast and HC wrought components, it was also observed that the delaminating layer was thinner at the apex of the wear zone, and thicker on the periphery.
4. Discussion Pitting on the cast and HC wrought components was most prominent on the implants tested to six million cycles. The fact that very little pitting was observed after three million cycles suggests that the micropits were formed by a fatigue mechanism. Although pitting through fatigue is generally characteristic of metals in rolling contact w18x and the hip implants were subjected to sliding contact, fatigue cannot be discounted as a contributing factor because the implants were subjected to cyclic loading. This is supported by the fact that the largest concentration of pitting was observed at the apex of the implants, where contact stress was greatest. The pattern of the micropitting differed between the two high carbon alloys. With the cast alloy, the pitting appeared to be primarily confined to carbide sites. With the HC wrought alloy, the pits seemed to originate at carbide sites, and progressively grow into the matrix with increased cycles. Whether this was influenced by stress risers at carbide sites, matrix embrittlement from local work hardening or a fatigue mechanism, could not be ascertained.
The finding of carbides at different levels relative to the surface of the matrix is indicative of complex tribological events occurring during simulator testing. With the HC wrought alloy, carbides that were slightly protruding in the as-manufactured condition were below the matrix surface after simulator testing. This is contrary to intuition that carbides are harder than the surrounding matrix and would be expected to be more wear-resistant and therefore remain proud. The finding of subsurface carbides might be partially explained by the possibility that protruding carbides could come into contact with each other during sliding and simply shear off. It might also be related to the relative mechanical properties of the carbides and matrix, the latter being more capable of elastic deformation under load and the former being more prone to fracture because of increased brittleness. Brittle carbide fracture could occur below the matrix surface; AFM revealed that the carbides below the surface possessed relatively rough surfaces, an appearance consistent with fracture. With the cast alloy, carbides were found both proud of the matrix surface and subsurface after simulator testing, the proud carbides typically being those that were isolated as opposed to those grouped in chains. The grouped carbides may have had different mechanical properties and hence different load carrying properties than the isolated carbides due to their geometry. It is also possible that the isolated carbides possessed a different composition than the chain carbides and thus behaved differently in the wear process. Both M 7 C 3 and M 23 C 6 carbides have been observed in similar alloys w11x. Further work is required to characterize differences that might exist between the isolated and chain-like carbides. The evidence of material loss through a delamination process at the implant surface can possibly be explained by contact mechanics in metallurgy w18–20x. In wear couples, the contacting surfaces plastically deform under load w18– 20x. The deformation causes dislocations to be generated within the material. While dislocations at the free surfaces are eliminated, dislocations that are subsurface are driven by the higher shear stress just below the surface and progressively move until they coalesce at obstacles such as carbides andror manufacturing defects. This results in the formation of voids that progressively grow by fatigue as a result of the applied cyclic loading. Eventually, the agglomeration of voids assumes the form of a crack parallel to the surface, usually at a depth close to that of the maximum shear stress. Such cracks can propagate and form plate-like or sheets of wear particles w18–20x. This delamination theory is only applicable to low speed sliding surfaces in which the temperature of the interface does not rise appreciably w20x, conditions that are reasonably satisfied by the hip simulator testing. After six million cycles, both the HC wrought and cast alloys showed what appeared to be layers or sheets of material missing from the surface, hence the assumption that the matrix wear occurred through delamination. This, however, could not be
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confirmed by examining cross-sections in an attempt to locate subsurface cracks, as such an examination would have been destructive. It could also be argued that the layers described may be material transfer from the opposing surface. However, adhesive wear of these materials, which has been observed during pin-on-disk wear testing in dry conditions, appears as smeared material. This was not observed on the implant heads. If the layers that were presumed to be delaminating matrix were in fact smears, the carbide sites would be concealed by the smeared material. However, the carbides were exposed. To further confirm matrix wear through delamination, a separate study is underway to examine the wear particles produced by these implants and contained within the bovine serum lubricant used during testing. It is interesting to note that in the case of the HC wrought and cast alloys, the removal of material through delamination seemed to have occurred only where the carbides were below the surface. In fact, in the case of the HC wrought alloy, the depth of carbide relief was similar to the thickness of the layer in which removal appeared imminent. This suggests that carbides are instrumental in maintaining the structural integrity of the matrix. As they fracture or wear away, matrix support may be reduced and the likelihood of material loss by delamination may increase. The present study has thus provided a new level of understanding about the microscopic changes that occur at the surfaces of Co–Cr–Mo alloy self-bearing hip implants after simulator testing. Additional insight has also been gained into the process of micropitting, a feature previously described by both Park et al. w21x and Medley et al. w22x. An entirely new information has been generated concerning carbide morphology as a function of alloy type and matrix wear by an apparent delamination process. A clear message from this study is that increased cycles of testing resulted in increased microscopic damage to the articulating surfaces. The observations made in this study did not elucidate the important issue of mass loss experienced by the implants during simulator testing. Nonetheless, they may provide information about the type of wear particles released by the three different alloys, an issue that could have important biological implications.
5. Conclusions This study has shown that the metallurgical and tribological events taking place at the articulating surfaces of metal–metal hip implants are numerous and complex. It has been shown that pits are formed on the surface by full or partial carbide pull-out and fatigue. The carbides in the cast and HC wrought components were shown to start proud of the surface, and eventually wear below the surface of the matrix, suggesting that the carbides wear faster
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than the matrix. It has been proposed that the matrix of all three alloys wears through delamination, however, more detailed studies are necessary for confirmation of this mechanism. Acknowledgements Medical Research Council of Canada, National Science and Engineering Research Council of Canada, Wright Medical Technology. References w1x H.C. Amstutz, P. Grigoris, Clinical Orthopaedics and Related Research 329 Ž1996. S11–S34, Suppl. w2x M.B. Schmidt, M.E. Lunn, Transactions of the Orthopaedic Research Society 23 Ž1998. 421. w3x R.M. Streicher, R. Schon, M. Semlitsch, Biomedizinische Technik 35 Ž1990. 107–111. w4x R.M. Streicher, Journal of Arthroplasty 13 Ž1998. 343–345. w5x B.G. Weber, Clinical Orthopaedics and Related Research 329 Ž1996. S69–S77, Suppl. w6x P.S. Walker, B.L. Gold, Wear 17 Ž1971. 285–299. w7x J.A. Schey, Clinical Orthopaedics and Related Research 329 Ž1996. S115–S127, Suppl. w8x A. Wang, J.D. Bobyn, S. Yue, J.B. Medley, F.W. Chan, Cobalt-base alloys for biomedical applications, ASTm STP 1365, in: J.A. Disegi, R.L. Dennedy, R. Pilliar ŽEds.., American Society for Testing and Materials, 1999, in press. w9x M.B. Schmidt, H. Weber, R. Schon, Clinical Orthopaedics and Related Research 329 Ž1996. S35–S47, Suppl. w10x S. Atamert, J. Stekly, Surface Engineering 9 Ž3. Ž1993. 231–240. w11x C.T. Sims, Journal of Metals Ž1996. 27–42. w12x J.B. Medley, F.W. Chan, J.J. Krygier, J.D. Bobyn, Clinical Orthopaedics and Related Research 329 Ž1996. S148–S159, Suppl. w13x J.B. Medley, J.J. Krygier, J.D. Bobyn, F.W. Chan, M. Tanzer, Transactions of the Orthopaedic Research Society 20 Ž1995. 765. w14x J.P. Paul, Proceedings of the Institution of Mechanical Engineers 181F Ž1967. 8–15. w15x F.W. Chan, J.D. Bobyn, J.B. Medley, J.J. Krygier, S. Yue, M. Tanzer, Clinical Orthopaedics and Related Research 333 Ž1996. 96–107. w16x J.B. Medley, J.J. Krygier, J.B. Bobyn, F.W. Chan, A. Lippincott, M. Tanzer, Proceedings of the Institution of Mechanical Engineers, Journal of Engineering in Medicine 211 Ž1997. 89–100. w17x J.I. Goldstein, D.E. Newbury, P. Echlin, D.C. Joy, A.D. Romig, C.E. Lyman, C. Fiori, E. Lifshin, Scanning Electron Microscopy and X-Ray Microanalysis—A Text for Biologists, Materials Scientists, and Geologists, 2nd edn., Plenum, New York, 1992, pp. 201–203. w18x J.A. Williams, Engineering Tribology, Oxford Press, New York, 1994, p. 188. w19x R.D. Arnell, P.B. Davies, J. Halling, T.L. Whomes, Tribology— Principles and Design Applications, Macmillan, London, 1991, p. 77. w20x A.D. Sarkar, Friction and Wear, Academic Press, London, 1980, pp. 59–60. w21x S.H. Park, H. McKellop, B. Lu, F.W. Chan, Transactions of the 23rd Annual Meeting of the Society for Biomaterials, May 1997, p. 191, in press. w22x J.B. Medley, J.M. Dowling, R.A. Poggie, J.J. Krygier, J.D. Bobyn, in: J.J. Jacobs, T.L. Craig ŽEds.., ASTM STP 1346 ŽAlternative Bearing Surfaces in Total Joint Replacement., ASTM, Nov. 1998.
Surface characterization of metal-on-metal hip implants tested in a hip simulator A. Wang a
a,)
, S. Yue a , J.D. Bobyn b, F.W. Chan b, J.B. Medley
c
Metallurgical Engineering Department, McGill UniÕersity, MH Wong Building, 3610 UniÕersity St., Montreal, Quebec, H3A 2B2, Canada b Jo Miller Orthopaedic Research Laboratory, Montreal General Hospital and McGill UniÕersity, Montreal, Quebec, Canada c Mechanical Engineering Department, UniÕersity of Waterloo, Waterloo, Ontario, Canada
Abstract The purpose of this study was to characterize metallurgical and tribological events occurring at the articulating surfaces of metal–metal implants tested in a hip simulator in order to gain understanding of the wear characteristics of Co–Cr–Mo alloys. The surfaces of 12 implant heads, made of either cast, low carbon wrought, or high carbon wrought Co–Cr–Mo material, were examined using scanning electron and atomic force microscopes. Three of the implants were examined prior to simulator testing, three after three million cycles of testing, and six after six million cycles of testing. Initially the carbides in the cast and high carbon wrought components were proud of the surface. With testing, in the high carbon wrought components, the carbides were worn below the matrix surface and were also a source for micropits. In the cast components, some of the carbides remained proud of the surface, while others were worn below the matrix surface with increased test cycles. Some of the carbides in the cast alloy experienced partial or full pull-out, resulting in micropits. All three alloys showed evidence of matrix wear through a process resembling delamination in which layers of material appeared to be removed. q 1999 Elsevier Science S.A. All rights reserved. Keywords: Metal–metal hip implants; Surface characterization; Hip simulator
1. Introduction Metal–metal bearing surfaces in artificial hip replacement are increasingly gaining acceptance because of their excellent wear resistance w1x. Several North-American implant manufacturers have developed metal–metal prostheses for clinical trials w1,2x, while in Europe, over 60,000 metal–metal hip bearings have been implanted in the past decade w3–5x. Of the metals that are commonly used as biomaterials, cobalt–chromium–molybdenum ŽCo–Cr– Mo. alloys are the preferred materials for self-bearing applications w6,7x, and these alloys are classified by the American Society for Testing and Materials ŽASTM.. While the original metal–metal hip implants from three decades ago were typically made of cast Co–Cr–Mo alloy w6x, the modern generation implants are made of three different Co–Cr–Mo alloys. Each alloy possesses a different microstructure, and thus might experience different wear mechanisms and a different wear resistance. At present, the mechanisms of wear and the factors governing )
Corresponding author. Tel.: q1-514-398-4755 ext. 09511; fax: q1514-398-4492; e-mail: [email protected]
wear resistance of each alloy are not well understood. The purpose of the present study is to characterize metallurgical and tribological events that occur in regions of direct surface contact during hip simulator testing of metal–metal implants. However, the present study is confined to examining carbide morphology and behavior, as well as matrix wear mechanisms. Abrasion and, in particular, third-body abrasion is examined in another publication by Wang et al. w8x. Surface characterization will help in the understanding of the wear mechanisms of metal–metal hip implants and guide alloy selection. 2. Materials and methods The implants under investigation were manufactured from the following Co–Cr–Mo alloys: cast ŽASTM F7592., low carbon ŽLC. wrought ŽASTM F1537-94. or high carbon ŽHC. wrought ŽASTM F1537-94.. The ASTM composition standards for these alloys are given in Table 1. The typical carbon content of the LC wrought alloys is 0.05 wt.% and that of the cast and HC wrought is 0.2 wt.%. All three alloys have a large proportion of Cr, which is necessary in order to create a passive oxide film that
0043-1648r99r$ - see front matter q 1999 Elsevier Science S.A. All rights reserved. PII: S 0 0 4 3 - 1 6 4 8 Ž 9 8 . 0 0 3 8 4 - 6
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Table 1 ASTM composition standards for surgical implant applications Alloy
Composition Žwt.%. Cr
Mo C
Fe
Ni
Si
Mn N
Co
Cast F75-92 26–30 5–7 F 0.35 F 0.75 F1 F1 F1 F 0.25 Bal. Wrought 27–30 5–7 F 0.35 F 0.75 F1 F1 F1 0 Bal. F1537-94
contributes substantially to corrosion resistance w9x. The alloys also have 5–7 wt.% Mo for solid solution strengthening w10x. Carbon is added at levels of less than 0.35 wt.%, also for solid solution strengthening, but, more importantly, for the formation of carbides, which further strengthen the material and possibly improve wear resistance. The carbides present in the alloys are usually in the form of M 23 C 6 , where M is some combination of Co, Cr and Mo w11x. The microstructures of these alloys ŽFigs. 1 and 2. were obtained by sectioning three heads, the surfaces of which were not examined. These sections were ground and polished to a final polish of 0.05 mm alumina suspension. The polished surfaces were electroetched with 60% nitric acid in water kept at room temperature. The cast alloy was etched for a total of 12 s, the HC wrought for 18 s and the LC wrought for 20 s, all at 5 V. Prior to SEM examination, the microstructures were sputter coated with gold in order to render them conductive. In order to form the implants, the various alloys were ground and polished into spherical heads of 28 mm diameter and hemispherical acetabular cups, which, when mated with the heads, had a diametrical clearance of approximately 40–100 mm. For the present study, the articulating surfaces of 12 heads were examined. Three of the heads Žone of each alloy. were examined as manufactured Ži.e., without any hip simulator testing.. Three heads Žone of each alloy. had been tested in a model EW08 MMED hip simulator ŽMATCO, La Canada, CA. for three million cycles w12,13x, and six heads Žthree of the cast and three of the HC wrought. had been tested in the hip simulator for six million cycles. The hip simulator subjected the articulating surfaces to a biaxial rocking motion with a peak load of 2.1 kN applied in a Paul loading cycle at 1.13 Hz, approximating the resultant normal force during walking w14x. All tests were run in bovine serum containing antibacterial and antifungal agents w12,15x. The cycle motion of each wear station simulated about 468 of flexion-extension and involved multidirectional motion at the articulating surfaces as described by Medley et al. w16x. Testing methods and conditions are described in detail by Chan et al. w15x. Information on the specimens is summarized in Table 2, along with the mass loss resulting from simulator testing. The articulating surfaces of the heads were examined using a JEOL 840 scanning electron microscope ŽSEM. at
Fig. 1. Typical microstructure of the cast alloy obtained with SEM in secondary electron mode. Ža. At low power; Žb. region from the window in Ža.; Žc. region from the window in Žb..
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Fig. 2. Typical microstructures obtained with SEM in secondary electron mode. Ža. HC wrought alloy; Žb. LC wrought alloy.
accelerating voltages of 5 or 10 kV. The SEM was used for topographical and compositional imaging, as well as for qualitative elemental analysis using a NORAN I2 energy dispersive spectrometer ŽEDS. that was equipped with an ultra thin window for detection of low atomic number elements. The surfaces were prepared for examination by cleaning with acetone andror replica tape. They were not sputter coated with gold or carbon because they were to be subjected to further wear testing in the hip simulator. As a complementary analytical technique, a Digital Instruments Dimension 3100 atomic force microscope ŽAFM. was also used to analyze the heads. This enabled better resolution and quantitative topography to be performed.
3. Results All twelve heads displayed third body abrasive scratches as reported and discussed in detail by Wang et al. w8x. The Table 2 The mass loss experienced by each implant that was examined Alloy
Cycles of articulation Žmillion.
Mass loss Žmg.
Cast LC wrought HC wrought Cast LC wrought HC wrought Cast Cast Cast HC wrought HC wrought HC wrought
0 0 0 3 3 3 6 6 6 6 6 6
0 0 0 1.3 8.0 1.2 5.8 5.1 6.5 5.1 8.5 5.8
present paper, however, concentrates on carbide morphology and behavior, as well as matrix wear mechanisms Žaside from abrasive scratches.. The cast microstructure ŽFig. 1. was the coarsest of the three alloys, having grains with diameters of several hundreds of micrometers. Carbides outlined the grain boundaries and were also found within the grains. Those within the grains were either in chains or isolated. The carbide size ranged from about 1 to 10 mm in diameter. When wrought, the grains were reduced in diameter to 10 mm or less. The HC wrought alloy ŽFig. 2a. had carbides which ranged in diameter from about 1 to 5 mm and were found in abundance along the grain boundaries as well as within the grains. Unlike the cast alloy, the grain boundaries of the HC wrought alloy were not outlined by the carbides. Most of the carbides in the LC wrought alloy ŽFig. 2b. were less than 0.5 mm in diameter and were found at the grain boundaries as well as within the grains. Some carbides, about 1 mm in diameter, were also present and were noted only at the grain boundaries. When examining the surfaces of the implant heads using SEM accelerating voltages of 5 and 10 kV, the carbides in the cast and the HC wrought were apparent, while those in the LC wrought heads were not. Although the carbides in the LC wrought alloy could be seen in the microstructure ŽFig. 2b., those on the implant heads could not because the implants were not sputter-coated with gold and thus the required resolution could not be achieved. When subjected to EDS, the carbide regions were found to have carbon, chromium and molybdenum at levels higher than in the matrix, thus confirming the visual identification. 3.1. Prior to simulator testing Prior to testing, the SEM examination seemed to suggest that the detectable carbides in the as-manufactured
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Fig. 3. AFM images illustrating carbides on components prior to simulator testing. The darkest regions are deepest, lightest regions are most proud. Ža. Cast alloy, apex region. Note the proud nature of the carbide chain; Žb. HC wrought alloy, apex region. Note the proud isolated carbides and the surface scratches from the manufacturing process. Images were obtained in tapping mode and were flattened to remove curvature effect.
cast and HC wrought components were flush with the surface. However, AFM examination showed these carbides to be standing proud ŽFig. 3.. Cross-sectional analyses of the carbides showed them to be protruding several tens of nanometers on the cast component, and less than 10 nm on the HC wrought component. The carbides in the LC wrought component were not seen with the AFM. Unlike the SEM, resolution achieved with the AFM is sufficient to detect the small carbides in the LC wrought alloy. However, since they were not detected the carbides were probably flush with the surface.
bides were always darker because of a back-scattering electron effect w17x. This effect is lessened when the sample is coated with a heavy element, such as gold. That is the reason why the carbides in the sputter-coated microstructures appeared light ŽFigs. 1 and 2., while those in the uncoated implants appeared dark Že.g., Fig. 5.. The carbide behavior in the LC wrought head could not be determined because of insufficient resolution. At this stage, components of each alloy displayed matrix wear through a process that could be described as
3.2. After three million cycles of testing After three million cycles in the hip simulator, the cast implant experienced a total mass loss of 1.3 mg, the HC wrought implant 1.2 mg and the LC wrought implant 8.0 mg ŽTable 2.. The wear occurred in a zone at the apex of the implant head ŽFig. 4.. In the wear zone, all of the carbides in the HC wrought head and most of the carbides found in chains in the cast component were no longer protruding from the surface, but were present below the surface of the matrix. This indicated that the carbides had been worn away at a faster rate than the surrounding matrix, as illustrated for the HC wrought head ŽFig. 5.. It should be noted that on the cast and HC wrought heads, the carbides always appeared darker than the matrix during secondary electron imaging with the SEM, regardless of whether they were above, flush, or below the matrix. Despite the fact that secondary electron imaging primarily reveals topographical features, it is believed that the car-
Fig. 4. Illustration of hip implant components which were tested in the simulator and the apical wear zone on the head used for the microscopic analyses.
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Fig. 5. SEM micrograph of the apex region from a HC wrought component tested to three million cycles. Numerous carbides just below the surface are visible as well as an irregular boundary suggestive of matrix delamination. Image was obtained in secondary electron mode.
delamination. This type of wear appeared as a discontinuous matrix, displaying a fragmented upper layer that rested on a nonfragmented lower layer ŽFig. 5.. 3.3. After six million cycles of testing After six million cycles, the three HC wrought implants experienced a total mass loss of 5.1, 8.5, and 5.8 mg,
Fig. 7. SEM micrograph of the apex of a cast component tested to six million cycles. The pits within the carbide chain possess smooth edges whereas the pits at isolated carbide sites possess noticeably rougher edges. This may be related to carbide composition and mechanism of pit formation. Image was obtained in secondary electron mode.
respectively ŽTable 2.. The carbides in the HC wrought heads remained below the surface at the wear zone, similar to that which was observed after three million cycles. Carbide behavior in the LC wrought components could not be determined at this stage, as no LC wrought heads were
Fig. 6. Images illustrating a carbide chain on a cast component tested to six million cycles. The carbide labeled U serves to orient the chain on both images. Ža. SEM micrograph obtained in secondary electron mode. Subsurface as well as protruding carbides appearing dark; Žb. AFM image of the same chain in Ža. taken in tapping mode and flattened to remove curvature effect. The carbides within the chain are all subsurface and some are pitted. The light, isolated carbides outside the chain are proud of the surface.
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Fig. 8. Ža. AFM image of a large pit on a HC wrought component tested to six million cycles. Image was obtained in tapping mode and was flattened to remove curvature effect. Žb. Cross-sectional analysis of the pit indicating a width of 4.4 mm and a depth of 113 nm.
available for examination after six million cycles. Three cast implants were available for examination after six million cycles. It was observed that these three implants had different microstructures with different carbide distributions. Two of the implants had substantially more carbide chains than the third, which had mainly isolated carbides. The two implants with many chains experienced a weight loss of 5.8 and 5.1 mg, respectively, and the implant with mainly isolated carbides experienced a similar weight loss of 6.5 mg ŽTable 2.. At this stage, the carbides found in chains were not only present slightly below the matrix surface Žas was seen after three million cycles., but some were fractured and experienced partial pull-out, resulting in pits ŽFig. 6.. In the two implants that had multiple carbide chains, the isolated carbides rarely experienced pull-out. The implant that had mainly isolated carbides showed what appeared to be full pull-out of most of the isolated carbides ŽFig. 7.. However, the carbides that were present in chains on this implant displayed the kind of pitting seen on the other two implants ŽFig. 7.. The pits resulting from isolated carbide pull-out had considerably rougher edges than the pits resulting from the partial carbide pull-out in the carbide chains. After six million cycles of testing, pits were found on the HC wrought heads. The pits were found at the apex over an area of about 1–4 mm2 ŽFig. 8.. The diameters of these pits were of the same order of magnitude as the carbides Ž1–5 mm., but penetrated only to a depth of about 100–140 nm. The HC wrought implant that experienced the most weight loss Ž8.5 mg. had substantially more pits than did the other two HC wrought heads, some as large as
10 mm in diameter. The smaller pits were associated with carbides, suggesting that the pits originated at carbides. Since the depth of the pits was an order of magnitude smaller than the carbides, it is likely that carbides still existed at the bottom of the pits. As was observed after three million cycles of testing, the matrix still wore by delamination after six million cycles. The delaminating matrix was once again of two layers. In the HC wrought component, the upper, fragmented layer possessed carbides that were below the matrix surface. The carbides present in the nonfragmented, lower layer were flush with the surface of this layer. This
Fig. 9. SEM micrograph illustrating a zone of delamination at the periphery of the apical wear zone on a HC wrought component tested to six million cycles. Image was obtained in secondary electron mode.
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Fig. 10. SEM micrograph illustrating upper and lower delamination layers at the periphery of the apical wear zone on a cast component tested to six million cycles. Image was obtained in secondary electron mode.
carbide behavior is depicted in Fig. 9. AFM examination of the top layer revealed that the depth to which the carbides were below the top layer was similar to the thickness of this layer. The depth and thickness were found to be approximately 30–40 nm, respectively. In the cast component, the carbides were also believed to be below the surface of the upper layer, and flush with the surface of the lower layer ŽFig. 10.. On both the cast and HC wrought components, it was also observed that the delaminating layer was thinner at the apex of the wear zone, and thicker on the periphery.
4. Discussion Pitting on the cast and HC wrought components was most prominent on the implants tested to six million cycles. The fact that very little pitting was observed after three million cycles suggests that the micropits were formed by a fatigue mechanism. Although pitting through fatigue is generally characteristic of metals in rolling contact w18x and the hip implants were subjected to sliding contact, fatigue cannot be discounted as a contributing factor because the implants were subjected to cyclic loading. This is supported by the fact that the largest concentration of pitting was observed at the apex of the implants, where contact stress was greatest. The pattern of the micropitting differed between the two high carbon alloys. With the cast alloy, the pitting appeared to be primarily confined to carbide sites. With the HC wrought alloy, the pits seemed to originate at carbide sites, and progressively grow into the matrix with increased cycles. Whether this was influenced by stress risers at carbide sites, matrix embrittlement from local work hardening or a fatigue mechanism, could not be ascertained.
The finding of carbides at different levels relative to the surface of the matrix is indicative of complex tribological events occurring during simulator testing. With the HC wrought alloy, carbides that were slightly protruding in the as-manufactured condition were below the matrix surface after simulator testing. This is contrary to intuition that carbides are harder than the surrounding matrix and would be expected to be more wear-resistant and therefore remain proud. The finding of subsurface carbides might be partially explained by the possibility that protruding carbides could come into contact with each other during sliding and simply shear off. It might also be related to the relative mechanical properties of the carbides and matrix, the latter being more capable of elastic deformation under load and the former being more prone to fracture because of increased brittleness. Brittle carbide fracture could occur below the matrix surface; AFM revealed that the carbides below the surface possessed relatively rough surfaces, an appearance consistent with fracture. With the cast alloy, carbides were found both proud of the matrix surface and subsurface after simulator testing, the proud carbides typically being those that were isolated as opposed to those grouped in chains. The grouped carbides may have had different mechanical properties and hence different load carrying properties than the isolated carbides due to their geometry. It is also possible that the isolated carbides possessed a different composition than the chain carbides and thus behaved differently in the wear process. Both M 7 C 3 and M 23 C 6 carbides have been observed in similar alloys w11x. Further work is required to characterize differences that might exist between the isolated and chain-like carbides. The evidence of material loss through a delamination process at the implant surface can possibly be explained by contact mechanics in metallurgy w18–20x. In wear couples, the contacting surfaces plastically deform under load w18– 20x. The deformation causes dislocations to be generated within the material. While dislocations at the free surfaces are eliminated, dislocations that are subsurface are driven by the higher shear stress just below the surface and progressively move until they coalesce at obstacles such as carbides andror manufacturing defects. This results in the formation of voids that progressively grow by fatigue as a result of the applied cyclic loading. Eventually, the agglomeration of voids assumes the form of a crack parallel to the surface, usually at a depth close to that of the maximum shear stress. Such cracks can propagate and form plate-like or sheets of wear particles w18–20x. This delamination theory is only applicable to low speed sliding surfaces in which the temperature of the interface does not rise appreciably w20x, conditions that are reasonably satisfied by the hip simulator testing. After six million cycles, both the HC wrought and cast alloys showed what appeared to be layers or sheets of material missing from the surface, hence the assumption that the matrix wear occurred through delamination. This, however, could not be
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confirmed by examining cross-sections in an attempt to locate subsurface cracks, as such an examination would have been destructive. It could also be argued that the layers described may be material transfer from the opposing surface. However, adhesive wear of these materials, which has been observed during pin-on-disk wear testing in dry conditions, appears as smeared material. This was not observed on the implant heads. If the layers that were presumed to be delaminating matrix were in fact smears, the carbide sites would be concealed by the smeared material. However, the carbides were exposed. To further confirm matrix wear through delamination, a separate study is underway to examine the wear particles produced by these implants and contained within the bovine serum lubricant used during testing. It is interesting to note that in the case of the HC wrought and cast alloys, the removal of material through delamination seemed to have occurred only where the carbides were below the surface. In fact, in the case of the HC wrought alloy, the depth of carbide relief was similar to the thickness of the layer in which removal appeared imminent. This suggests that carbides are instrumental in maintaining the structural integrity of the matrix. As they fracture or wear away, matrix support may be reduced and the likelihood of material loss by delamination may increase. The present study has thus provided a new level of understanding about the microscopic changes that occur at the surfaces of Co–Cr–Mo alloy self-bearing hip implants after simulator testing. Additional insight has also been gained into the process of micropitting, a feature previously described by both Park et al. w21x and Medley et al. w22x. An entirely new information has been generated concerning carbide morphology as a function of alloy type and matrix wear by an apparent delamination process. A clear message from this study is that increased cycles of testing resulted in increased microscopic damage to the articulating surfaces. The observations made in this study did not elucidate the important issue of mass loss experienced by the implants during simulator testing. Nonetheless, they may provide information about the type of wear particles released by the three different alloys, an issue that could have important biological implications.
5. Conclusions This study has shown that the metallurgical and tribological events taking place at the articulating surfaces of metal–metal hip implants are numerous and complex. It has been shown that pits are formed on the surface by full or partial carbide pull-out and fatigue. The carbides in the cast and HC wrought components were shown to start proud of the surface, and eventually wear below the surface of the matrix, suggesting that the carbides wear faster
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than the matrix. It has been proposed that the matrix of all three alloys wears through delamination, however, more detailed studies are necessary for confirmation of this mechanism. Acknowledgements Medical Research Council of Canada, National Science and Engineering Research Council of Canada, Wright Medical Technology. References w1x H.C. Amstutz, P. Grigoris, Clinical Orthopaedics and Related Research 329 Ž1996. S11–S34, Suppl. w2x M.B. Schmidt, M.E. Lunn, Transactions of the Orthopaedic Research Society 23 Ž1998. 421. w3x R.M. Streicher, R. Schon, M. Semlitsch, Biomedizinische Technik 35 Ž1990. 107–111. w4x R.M. Streicher, Journal of Arthroplasty 13 Ž1998. 343–345. w5x B.G. Weber, Clinical Orthopaedics and Related Research 329 Ž1996. S69–S77, Suppl. w6x P.S. Walker, B.L. Gold, Wear 17 Ž1971. 285–299. w7x J.A. Schey, Clinical Orthopaedics and Related Research 329 Ž1996. S115–S127, Suppl. w8x A. Wang, J.D. Bobyn, S. Yue, J.B. Medley, F.W. Chan, Cobalt-base alloys for biomedical applications, ASTm STP 1365, in: J.A. Disegi, R.L. Dennedy, R. Pilliar ŽEds.., American Society for Testing and Materials, 1999, in press. w9x M.B. Schmidt, H. Weber, R. Schon, Clinical Orthopaedics and Related Research 329 Ž1996. S35–S47, Suppl. w10x S. Atamert, J. Stekly, Surface Engineering 9 Ž3. Ž1993. 231–240. w11x C.T. Sims, Journal of Metals Ž1996. 27–42. w12x J.B. Medley, F.W. Chan, J.J. Krygier, J.D. Bobyn, Clinical Orthopaedics and Related Research 329 Ž1996. S148–S159, Suppl. w13x J.B. Medley, J.J. Krygier, J.D. Bobyn, F.W. Chan, M. Tanzer, Transactions of the Orthopaedic Research Society 20 Ž1995. 765. w14x J.P. Paul, Proceedings of the Institution of Mechanical Engineers 181F Ž1967. 8–15. w15x F.W. Chan, J.D. Bobyn, J.B. Medley, J.J. Krygier, S. Yue, M. Tanzer, Clinical Orthopaedics and Related Research 333 Ž1996. 96–107. w16x J.B. Medley, J.J. Krygier, J.B. Bobyn, F.W. Chan, A. Lippincott, M. Tanzer, Proceedings of the Institution of Mechanical Engineers, Journal of Engineering in Medicine 211 Ž1997. 89–100. w17x J.I. Goldstein, D.E. Newbury, P. Echlin, D.C. Joy, A.D. Romig, C.E. Lyman, C. Fiori, E. Lifshin, Scanning Electron Microscopy and X-Ray Microanalysis—A Text for Biologists, Materials Scientists, and Geologists, 2nd edn., Plenum, New York, 1992, pp. 201–203. w18x J.A. Williams, Engineering Tribology, Oxford Press, New York, 1994, p. 188. w19x R.D. Arnell, P.B. Davies, J. Halling, T.L. Whomes, Tribology— Principles and Design Applications, Macmillan, London, 1991, p. 77. w20x A.D. Sarkar, Friction and Wear, Academic Press, London, 1980, pp. 59–60. w21x S.H. Park, H. McKellop, B. Lu, F.W. Chan, Transactions of the 23rd Annual Meeting of the Society for Biomaterials, May 1997, p. 191, in press. w22x J.B. Medley, J.M. Dowling, R.A. Poggie, J.J. Krygier, J.D. Bobyn, in: J.J. Jacobs, T.L. Craig ŽEds.., ASTM STP 1346 ŽAlternative Bearing Surfaces in Total Joint Replacement., ASTM, Nov. 1998.