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The effect of a novel CoCr electropolishing technique on CoCr-UHMWPE bearing frictional performance for total joint replacements
Tribology International 47 (2012) 204–211
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Tribology International journal homepage: www.elsevier.com/locate/triboint
The effect of a novel CoCr electropolishing technique on CoCr-UHMWPE bearing frictional performance for total joint replacements Alvarez Estefaniaa, Vinciguerra Johnb, DesJardins John Da,n a b
Clemson University, Bioengineering Department, 301 Rhodes Research Center, Clemson, SC 29634, USA DJO Surgical, Austin, TX, USA
a r t i c l e i n f o
a b s t r a c t
Article history: Received 8 January 2011 Received in revised form 6 June 2011 Accepted 1 November 2011 Available online 29 November 2011
This study quantified the effect of a novel electropolishing treatment (Ultra PolishTM) on CoCr surface morphology and coefficients of friction (CoF) when slid against UHMWPE pins. Standard and Ultra Polish CoCr samples were characterized using non-contact surface profilometry and SEM. The Ultra Polish process showed aggressive surface carbide removal, leaving behind large randomized pitted areas on CoCr. The trading of carbide peaks for pitted valleys, resulted no statistical change in surface roughness average (Ra) or dynamic CoF between Standard and Ultra Polish Treated groups. Mean Valley Roughness (Rvm) was better able to quantify changes in surface morphology seen in this study. & 2011 Elsevier Ltd. All rights reserved.
Keywords: Total joint replacement CoCr Coefficient of friction Polishing
1. Introduction Over the past forty years, the coupling of a cobalt chrome molybdenum (CoCr) metal femoral component with an ultra high molecular weight polyethylene (UHMWPE) component has become the ‘gold’ standard for a total joint replacement due to its low revision rates and low coefficient of friction [1–5]. Within this load bearing couple, CoCr alloys have been considered as the traditional femoral component material due to its excellent corrosion resistance, wear resistance, high fatigue strength and its ease for processing. Unfortunately, the accumulation of micron size UHMWPE wear particles is cited as triggering an adverse osteolytic biological response which eventually leads to aseptic loosening of the implant and subsequent significant reduction in the implant’s longevity [6]. The use of hard scratch resistant coatings and the use of crosslinked UHMWPE have been shown to improve the wear performance in this bearing couple [7–13]. Within metal–poly and metal–metal bearing systems, the material wear performance of metal alloys has been linked to the carbon content and heat treatment of the alloy used [14]. These two parameters contribute to the carbide formation and eventual protrusion, which is responsible for scratching and delamination of the UHMWPE [15–18]. CoCr alloys have two phases: cobalt alloy solid solution matrix and metal carbides. The two commercially available CoCr formulations, forged and as cast,
n
Corresponding author. E-mail address: [email protected] (J. DesJardins).
0301-679X/$ - see front matter & 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.triboint.2011.11.002
mainly differ on the carbon content as well as the distribution and morphology of the carbides. The forged CoCr (low C content approximately 0.067% weight) is known to have a coarse distribution of carbides along the surface. The high carbon cast CoCr (ASTM F75 with approximately 0.26% weight) tends to be problematic to control grain size, carbide morphology and distribution [19]. Carbides are known to protrude from the surface of CoCr alloys by as much as 0.2 mm [17,18]. They are high hardness second phase materials that are associated not only with accelerated wear of the softer material of the bearing but also known to strengthen the alloy matrix [14]. Several studies report that the amount of carbon, carbide volume fraction and carbide morphology have an effect on the wear resistance on metal on metal THR tested under simulated physiological conditions [14,20–22]. Similar to the current study, these investigations focused on quantifying the effect of a novel surface treatment on the surface morphology and the resulting frictional behavior between novel material pairings. To reduce the carbide concentration and protrusion at the surface of total joint implant surfaces, a surface engineering treatment can be used to treat and polish the CoCr femoral components. An example of a surface treatment is the Ultra Polish treatment which uses a patented electro polishing treatment [23] to reduce the height and concentration of the carbides by submerging the sample in an electrolytic bath solution with the proper current density used to produce a fine smooth surface polish [24]. The treatment has been cited to reduce the height and concentration of the carbides that is hoped to lead to a reduction in bearing friction between the components. This treatment can
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of Ultra Polish Treated CoCr vs. Standard CoCr surfaces against medical grade standard UHMWPE. A set of six gas plasma sterilized and cleaned compression molded GUR 1050 (non-crosslinked) UHMWPE pin specimens (1.25 in length, 3/8 in dia. tapered bearing tip of 4.2 mm with 301 of tapered angle with Ra¼3339.572666 nm per ASTM 2083) were used. The Ultra Polish Treated and Standard CoCr test disks were approximately 170.02 in diameter with thickness of 3/16 in with surface roughness finish that meets ASTM F732 (ranging from 0.026 to 0.05 mm) commonly used in the orthopedic device industry [27], with a typical carbon content for medical grade CoCr of 0.27 wt%. Tests were conducted under a normal load of 50 N and a frequency of 1 Hz which produced a relative maximum sliding velocity of 35 mm/s against the CoCr counterface with a total traveled distance of 1 km (30,000 cycles) [28]. This compressive load yielded a nominal contact of 3.6 MPa, which lies within the appropriate physiological range for a hip joint (1–3 MPa) [28–31]. The displacement pathway traced an arc of length of 17.4 mm using a multi-directional six station pin-on-disk machine wear tester OrthopodTM (Advanced Mechanical Technology Inc, Waltham, MA). The effect of the polishing procedure on the surface of the CoCr disks was evaluated using non contact profilometry using a WYKO NT2000 profilometer (Veeco Corp., Tucson, AZ). Surface characterization was performed before and after frictional testing at a nominal magnification of 25 (Field of view 736 480 nm, 70.1 nm). All surfaces were characterized for roughness measures arithmetic surface roughness (Ra), root mean squared roughness (Rq), roughness peak mean (Rpm) and roughness valley mean (Rvm). Measurements were taken at 8 points within the locations of expected frictional testing on each disk in a circular fashion in order to fully quantify and characterize the component and ensure reliable and repeatable estimate of surface roughness. All surface roughness analyses were filtered to compensate for macroscopic measures of surface geometry only (tilt and curvature). Statistical analysis (student’s t-test with p¼ 0.05) was performed to evaluate whether there was a significant difference between Standard and Ultra Polish Treated coefficients of friction and surface roughness parameters. Further 2D X–Y tracing was performed on selected areas of interest on both CoCr disks using the surface profilometer to assess discrete peak and valley architectures. To monitor for any unexpected material loss or damage of the UHMWPE pins, their weights were recorded both (Satorius,
be performed on CoCr alloys as they are susceptible to work hardening and finishing processes. Therefore, this surface treatment on the counterface of the polymeric/metal coupling could potentially extend the longevity of the implant. In this study, a short-run reciprocating pin-on-disk friction test was performed between cast CoCr disks (Standard CoCr and Ultra Polish Treated CoCr) slid against standard non-crosslinked ultrahigh molecular weight polyethylene (UHMWPE) pins under standard pin on disk conditions modeled after total joint replacement tribological conditions. This study focused on the evaluation of the CoF for a industry standard material pairing used in THR to assess the effectiveness of this novel surface treatment (Ultra Polish) to not only reduce the incidence of carbide protrusion but also to reduce CoCr-UHMWPE bearing coefficient of friction (CoF).
2. Materials and methods Two reciprocating pin-on-disk friction tests were conducted in series to evaluate and quantify the dynamic friction performance
Dynamic Coefficient of Friction
0.12 0.1 0.08 0.06 0.04
Ultra Polish Treated CoCr Standard CoCr
0.02
Ultra Polish Treated CoCr Standard CoCr
0 0
5000
10000
15000
20000
25000
205
30000
Cycles Fig. 1. Dynamic frictional behavior of Standard and Ultra Polish Treated CoCr disks slid against UHMWPE as a function of cycles traveled (Diluted Bovine Serum at 37 1C).
Table 1 Values of the roughness parameters for both Standard and Ultra Polish Treated CoCr disks before 30,000 cycles of articulations against UHMWPE. (Diluted Bovine Serum at 37 1C). Standard CoCr disks
Mean 7Std. Dev (nm) Max. Min.
Ultra polish CoCr disks
Ra
Rvm
Rpm
Ra
Rvm
Rpm
31.8 78.6 51.7 18.3
364.8 7 199.6 135.9 1162.7
363.9 7 175.9 1114.4 158.9
29.07 8.7 57.1 17.1
722.7 7 295.2 264.9 1412.0
388.5 7 98.4 1114.4 158.9
Table 2 Values of the roughness parameters for both Standard and Ultra Polish Treated CoCr disks after 30,000 cycles of articulations against UHMWPE. (Diluted Bovine Serum at 37 1C). Standard CoCr disks
Mean 7Std. Dev (nm) Max. Min.
Ultra polish CoCr disks
Ra
Rvm
Rpm
Ra
Rvm
Rpm
36 7 13.2 92.80 18.30
376.6 7 203.4 135.90 1162.70
363.9 7175.9 1114.40 158.90
49.6 7 46.4 196.70 16.50
730.77 294.8 264.90 1428.80
387.2 7 97.7 736.90 226.10
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Bohemia, NY, which has an accuracy of 70.0001 g) before and after the 1 km test. Prior to testing, all the samples (CoCr disks, disk holders) were ultrasonically cleaned following ASTM F1715 [32]. During testing, the planar dynamic coefficient of friction for each pin-disk pair was monitored. The coefficient of friction was
p = 0.42
500
Surface Parameters (nm)
300 p = 0.053
100
Rvm Ra
-100
Rpm
-300 -500 Standard CoCr Ultra Polish Treated CoCr
-700 -900 -1100
p < 0.0005*
Fig. 2. Roughness parameters of Standard and Ultra Polish Treated CoCr disks after 30,000 cycles of articulation against UHMWPE. (Diluted Bovine Serum at 37 1C).
collected every 500 repetitions for all six stations with a data acquisition frequency of 200 points/s. Testing was conducted using 25% bovine serum (Hyclone, Logan, UT) and 0.2% sodium azide (NaN3, Fisher Scientific: S227-100) at 37 1C 72 1C following previous studies and according to ASTM F1715-96 [27,33,34]. Sodium azide was used to limit any bacterial growth. The lubricant solution was non circulating in each station and it was kept at 37 1C using a heated water bath around the all the stations. This lubricant was replenished with deionized water as evaporation occurred, and the same lubricant solution was used for the length of the study. At the conclusion of 30,000 cycles of testing, a statistical analysis (student’s t-test with p¼ 0.05) of frictional coefficients for all pins was conducted to determine the effectiveness of the Ultra Polish process to reduce the friction between the polymer/metal couple. At the end of the test, non contact profilometry was performed on both sample groups.
3. Results Fig. 1 presents the six-station means of the dynamic coefficient of friction and their respective standard deviation as a function of sliding cycles for the Ultra Polish Treated and Standard CoCr against UHMWPE. For both groups the initial coefficient of friction is relatively high and then stabilizes over time. The data was fitted to a polynomial trend line of second order, which led to the highest correlation in both cases. No statistical differences were found between the coefficients of friction between material pairs
Fig. 3. Surface profile of an as cast Standard CoCr specimen when slid against non crosslinked UHMWPE under physiological conditions after 30,000 cycles (Diluted Bovine Serum at 37 1C). Significant carbide protrusion can be seen from the alloy surface. (For interpretation of the references to colour in this figure, the reader is referred to the web version of this article.)
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after 30,000 cycles. A power analysis (n¼6, Std. Dev¼0.0137, a ¼0.81) determined that the test enabled accurate and reliable statistical judgments able to detect shifts in the mean down to 0.025. Statistical analysis determined that both the Ultra Polish Treated and Standard disks have a normal distribution of the coefficient of friction after 30,000 cycles (p40.05, Ultra Polish Treated p ¼0.757, Standard CoCr p¼ 0.55). By inspection of both Standard and Ultra Polish Treated disks, there was no visual evidence of a wear track found after the sliding contact of UHMWPE for 1 km. For the CoCr surfaces, no statistical differences in the average surface roughness (Ra) or average maximum profile peaks (Rpm) were found between the Standard and Ultra Polish groups before or after testing, refer to Table 1 and 2. However, maximum profile valley height (Rvm) yielded statistically significant differences (p o0.0005), refer to Table 2. The average maximum profile depth for the Ultra Polish Treated disks was almost doubled the value of the Standard CoCr disks. The Ultra Polish Treatment showed evidence of extracted carbides leaving behind asymmetrical elongated ‘pitted’ regions. A typical representation of the ‘removed’ carbide area is represented on Fig. 6 and the subsequent 2D analysis that shows the typical size in X, Y coordinates of the removed carbide area. Five measurements were taken for three disks to approximate the dimension of the removed carbide area, and found to be of an approximate width of 5.21 71.01 mm and depth of 1.53 70.32 mm. The length was not quantified as it is of an amorphous elongated shape that could possibly incorporate more than one removed carbide area. Fig. 3 presents several surface roughness profiles of the Standard CoCr disks after 30,000 cycles of articulation. These
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profiles show the carbide protrusion (highlighted in dark red and shown with an arrow) distributed randomly across the surface with a maximum average height across all specimens (n¼ 6) measured being 719.9 71003.8 nm for the Standard CoCr disks. Using 2D surface analysis, the heights and widths of five representative CoCr carbide protrusions from each of 3 disks were measured and found to be 11.971.4 mm and 0.1970.01 mm, respectively. (Figs. 2 and 3). Fig. 4 presents a typical carbide feature on the surface of the Standard CoCr disks. The point of interest refers to the highlighted dark red region. This procedure was repeated to quantify the other features on the surface and used to validate the typical height and extension of the carbide colonies, similar to the measures reported in literature [14,16]. The topography of the Ultra Polish Treated disks consists of randomly distributed large and deep pits (highlighted as navy blue bright yellow and shown with an asterisk) seen in Fig. 5. These regions appear to be organized following the grain boundaries of the metal alloy. Other studies have reported similar surface profiles for as cast (ASTM polished) CoCr disks [18]. Maximum average surface heights of the Ultra Polish Treated disks were found to be 545.7 7226.9 nm. Fig. 6 presents a typical pitted area featured on the surface of the Ultra Polish Treated CoCr disks. Five measurements were taken for three Ultra Polish Treated CoCr disks using the 2D analysis to approximate the dimensions of both the width and depth of the pits. The width of the pitted areas were approximately 5.2171.01 mm with a depth of approximately 0.5870.09 mm. The surface topography was also characterized using the SEM as seen in Fig. 7. The carbide colonies are highlighted by the
Fig. 4. (A) Surface profile of a standard CoCr disk articulated against UHMWPE at 30,000 cycles. The cross point represents a selected area of interest it represents the carbide colony protruding through the surface. Typical 2D representative analysis of a selected area of interest (cross point) of an (B) X axis plot showing the width of the area 7 mm and (C) Y axis plot showing the height of the carbide feature 0.3 mm.
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Fig. 5. Representative surface profile of an Ultra Polish Treated CoCr alloy disk, which shows pitted areas as a result of the extraction due to the Ultra Polish procedure. (28.7 ). (For interpretation of the references to colour in this figure, the reader is referred to the web version of this article.)
arrows. These same colonies refer to the highlighted distributed red elongated features in Fig. 3. On the other hand, the removed carbide regions are shown in Fig. 8 for the Ultra Polish Treated CoCr disks. Both low and high magnification photomicrographs illustrate that the removed carbides do not only account for the carbide itself but the Ultra Polish treatment appears to extract the surrounding region as well. The Ultra Polish treatment creates a surface covered with irregular shaped elongated pitted regions. Post polishing or etching of the Ultra Polish Treated surfaces is evident with smoothed pit edges apparent.
4. Discussion The experimental results of this study show that the reported dynamic coefficient of friction for both Standard and Ultra Polish treated disks paired to UHMWPE fall within the values reported in the literature for similar material pairs used in total joint replacements. This study used a similar reciprocating arc pin on disk ASTM standard method used in total joint prosthesis when subjected to physiological loading conditions [35]. The typical traditional reciprocating pin on disk involved the stationary pin with the sliding disk, which created a linear wear path of 50 mm [36]. Due to initial manufacturing requirements as part of the Ultra Polish treatment process, the CoCr disks had small hole in their centers to secure the specimens during polishing. Full scale use of this polishing process on total joint replacement devices would require the manufacture of custom tooling and fixturing during polishing. This study used test parameters that followed ASTM standards: frequency of
1 Hz and maximum sliding velocity of 35 mm/s. The sliding velocity used in this test was within the expected sliding velocity used in reciprocating pin on disk tests for total joint materials (0–50 mm/s as for total joints) [28,37–40]. This study was designed based on previous studies that have used similar test parameters based on the ASTM F732-82 [37]. Saikko et al. developed a three station reciprocating pin on disk test equipment to evaluate the coefficient of friction and wear rates of typical total joint materials such as UHMWPE, CoCr, Al2O3, ZrO2 and Si3N4 under physiological conditions (nominal contact pressure of 4.8 MPa and de-ionized water as lubricant at 1 Hz) [36]. The test conditions involved sliding distance of 50 mm over 7 106 cycles. Previous studies using a reciprocating pin on disk apparatus have reported coefficients of friction between UHMWPE and CoCr under physiological conditions at contact pressures simulating a typical hip replacement (approximately 3 MPa) ranging from 0.03 to 0.15 [29,31,36,41,42]. Quantifying surface roughness parameters allows the possibility not only to evaluate the effect of surface roughness on the friction in a traditional material pairing for total hip joints, but it also enables a better understanding of potential applications for metal on metal total joint replacements. Other studies have only considered the arithmetic mean roughness (Ra) to be directly correlated to the friction when comparing two material pairings. However, in this study there is no significant differentiation between the Ra of Standard and Ultra Polish Treated CoCr disks. This parameter only accounts for the absolute mean values of the measured peaks with relation to the surface, therefore is the main height calculated overall the entire measured length. However, this does not take into account any spatial and textural variation of the topography, which means
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Fig. 6. (A) Surface profile of an Ultra Polish Treated CoCr disk articulated against UHMWPE at 30,000 cycles. The cross point represents a selected area of interest it represents the removed area after the Ultra Polish Treatment. Typical 2D representative analysis of a selected area of interest (cross point) of an (B) X axis plot showing the width of the area 5 mm and (C) Y axis plot showing the depth of the area of interest 0.6 mm.
Fig. 7. Surface microstructure of the Standard CoCr disks after articulation against UHMWPE for 30,000 cycles. (A) 200 , (B) 300 and (C) 300 . The feature highlighted with the arrow is a typical representation of the carbide colony arranged along the grain boundaries protruding towards the surface.
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Fig. 8. Surface microstructure of the Ultra Polish Treated CoCr disks after articulation against UHMWPE for 30,000 cycles. (A) 110 , (B) 230 and (C) 3000 . The feature highlighted with the arrow is a typical representation of the removed carbide region distributed across the entire surface, which is shown in (C) at higher magnification.
that the significant removal of material using the Ultra Polish Treatment does not weigh heavily on this parameter. Therefore, there is need to consider the surface parameters such as Rpm and Rvm that consider the average peaks and average valleys, respectively and represent a more robust value that accounts for the average of several peak heights and valley depths. The Ultra Polish Treated CoCr disks had an average Rvm almost double of what the Standard CoCr disks considering several amorphous shaped pits. The variability in size and depth of these pits is also evidenced in the standard deviation roughness values of the Ultra Polish Treated disks. The material removal was initially hypothesized to be beneficial in order to lower the coefficient of friction. However, these pits create a large surface defect area which could be hypothesized to have a plowing effect that could be a source for abrasive wear and possibly scratching the UHMWPE due to continuous contact. The removal of the hard elastic carbide asperities is presumably intended to reduce the interlocking of asperities that can contribute to increases in friction. Although the carbide removal is evident, it was determined in this study that the process did not result in a significant lowering of the friction coefficient. Rpm represents a better indicator of the degree of pile up since it is the average of the profile peaks through the entire surface and to better characterize the surface finish [43,44]. This pile up is an important parameter that must be considered as it has been shown that even a small increase in this measure (from 0.2 to 2.3 mm) can lead to a significant increase in UHMWPE wear during in vitro studies [45]. Therefore, accounting only for Rp and Rv might not be a representative value as they are sensitive to high peaks and deep valleys and could be a representation of outliers that could not be the true representations of the surface conditions. The effect of the Ultra Polish treatment was intended to reduce the coefficient of friction through the production of a smoother ‘‘carbide free’’ surface. However, this study indicates that there is no significant difference between the coefficients of friction of both groups against UHMWPE. The Ultra Polish treatment was responsible for the removal of not only the carbide but also a
significant zone surrounding the carbide, thus creating a pitted area that could have had an effect on the contact between the articulating components. The lack of statistical difference in the CoF could be also linked to a slight difference in the processing of both tested groups. In the case of the Ultra Polish Treated CoCr, pre-polishing (i.e., hand polishing) was performed before the actual Ultra Polish Treatment. The Standard CoCr disks were ‘polished’ under ASTM F732 standards and did not go through the Ultra Polish Treatment. This difference in ‘pre-treatment’ could have affected the final roughness values, and thus resulting frictional data. However, the pre-polishing would not have been expected to be able to alter the Ultra Polish pitting effect, thus that would remain. A full scale wear test could help determine whether there are significant differences in long term friction and wear between both groups and determine which surface artifact has a larger impact on the coefficient of friction: carbide protrusion in the Standard CoCr disks or the significant pits formed after the Ultra Polish Treatment. The process of carbide removal from metal bearing components could be extremely beneficial for future applications in metal on metal total joints or resurfacing techniques. The removed ‘carbide’ area could serve to trigger an entrapment mechanism for the wear particles as well as a source for continuous lubrication. The ideal lubrication mode in total joint replacements could be considered to be a fluid film regime to promote a load transmitting fluid layer between two solid materials. Possible future studies could include the study of the lubrication regime for Ultra Polish Treated metal on metal joints. Considering that the nature of the lubricating regime is based on the ratio of the thickness of the lubricant film and the surface roughness of the surfaces, the effect of the Ultra Polish Treatment on a metal on metal THR could be significant in maintaining a constant fluid film during lubrication, which would be an significant improvement to the boundary-mixed lubrication regime that is reported in a Metal on Polyethylene THR. However, when
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considering the low wear combination of metal on metal, previous studies do consider that the metallic wear debris and ion release can bring a greater risk to develop hypersensitivity, cytotoxicity and theoretically risk of carcinogenesis for prolonged exposure [46]. It is important to acknowledge that the surface finish of the components can have an effect not only on the friction but also on the damage and wear of the bearing materials. Conversely, if surfaces wear or become damaged, this geometric change can have an reciprocal effect on the lubrication mode. This complex symbiotic relationship between friction, wear and lubrication is best studied under the specific design conditions envisioned for the final bearing assembly and are often modeled computationally. Preliminary measures of surface architecture and coefficients of friction, such as the ones presented in the current work, provide invaluable data to these models for the better design and evaluation of these devices.
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Wear and friction properties of prosthetic joint materials evaluated on a reciprocating pin-on-flat apparatus. Wear 1993;166(2):169–78. [37] Evaluation of Friction and Wear Properties of Polymeric Materials for Use in Total Joint Prostheses Using Micro-Tribometer Mod., Astm Designation F 732–82. American Society for Testing and Materials, Philadelphia, Pa. [38] Fisher J, Dowson D, Hamdzah H, Lee HL. The effect of sliding velocity on the friction and wear of Uhmwpe for use in total artificial joints. Wear 1994;175(1–2):219–25. [39] Gevaert MR, LaBerge M, Gordon JM, DesJardins JD. The quantification of physiologically relevant cross-shear wear phenomena on orthopedic bearing materials using the max-shear wear testing system. Journal of TribologyTransactions of the Asme 2005;127(4):740–9. [40] Klapperich C, Komvopoulos K, Pruitt L. Tribological properties and microstructure evolution of ultra-high molecular weight polyethylene. 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Contents lists available at SciVerse ScienceDirect
Tribology International journal homepage: www.elsevier.com/locate/triboint
The effect of a novel CoCr electropolishing technique on CoCr-UHMWPE bearing frictional performance for total joint replacements Alvarez Estefaniaa, Vinciguerra Johnb, DesJardins John Da,n a b
Clemson University, Bioengineering Department, 301 Rhodes Research Center, Clemson, SC 29634, USA DJO Surgical, Austin, TX, USA
a r t i c l e i n f o
a b s t r a c t
Article history: Received 8 January 2011 Received in revised form 6 June 2011 Accepted 1 November 2011 Available online 29 November 2011
This study quantified the effect of a novel electropolishing treatment (Ultra PolishTM) on CoCr surface morphology and coefficients of friction (CoF) when slid against UHMWPE pins. Standard and Ultra Polish CoCr samples were characterized using non-contact surface profilometry and SEM. The Ultra Polish process showed aggressive surface carbide removal, leaving behind large randomized pitted areas on CoCr. The trading of carbide peaks for pitted valleys, resulted no statistical change in surface roughness average (Ra) or dynamic CoF between Standard and Ultra Polish Treated groups. Mean Valley Roughness (Rvm) was better able to quantify changes in surface morphology seen in this study. & 2011 Elsevier Ltd. All rights reserved.
Keywords: Total joint replacement CoCr Coefficient of friction Polishing
1. Introduction Over the past forty years, the coupling of a cobalt chrome molybdenum (CoCr) metal femoral component with an ultra high molecular weight polyethylene (UHMWPE) component has become the ‘gold’ standard for a total joint replacement due to its low revision rates and low coefficient of friction [1–5]. Within this load bearing couple, CoCr alloys have been considered as the traditional femoral component material due to its excellent corrosion resistance, wear resistance, high fatigue strength and its ease for processing. Unfortunately, the accumulation of micron size UHMWPE wear particles is cited as triggering an adverse osteolytic biological response which eventually leads to aseptic loosening of the implant and subsequent significant reduction in the implant’s longevity [6]. The use of hard scratch resistant coatings and the use of crosslinked UHMWPE have been shown to improve the wear performance in this bearing couple [7–13]. Within metal–poly and metal–metal bearing systems, the material wear performance of metal alloys has been linked to the carbon content and heat treatment of the alloy used [14]. These two parameters contribute to the carbide formation and eventual protrusion, which is responsible for scratching and delamination of the UHMWPE [15–18]. CoCr alloys have two phases: cobalt alloy solid solution matrix and metal carbides. The two commercially available CoCr formulations, forged and as cast,
n
Corresponding author. E-mail address: [email protected] (J. DesJardins).
0301-679X/$ - see front matter & 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.triboint.2011.11.002
mainly differ on the carbon content as well as the distribution and morphology of the carbides. The forged CoCr (low C content approximately 0.067% weight) is known to have a coarse distribution of carbides along the surface. The high carbon cast CoCr (ASTM F75 with approximately 0.26% weight) tends to be problematic to control grain size, carbide morphology and distribution [19]. Carbides are known to protrude from the surface of CoCr alloys by as much as 0.2 mm [17,18]. They are high hardness second phase materials that are associated not only with accelerated wear of the softer material of the bearing but also known to strengthen the alloy matrix [14]. Several studies report that the amount of carbon, carbide volume fraction and carbide morphology have an effect on the wear resistance on metal on metal THR tested under simulated physiological conditions [14,20–22]. Similar to the current study, these investigations focused on quantifying the effect of a novel surface treatment on the surface morphology and the resulting frictional behavior between novel material pairings. To reduce the carbide concentration and protrusion at the surface of total joint implant surfaces, a surface engineering treatment can be used to treat and polish the CoCr femoral components. An example of a surface treatment is the Ultra Polish treatment which uses a patented electro polishing treatment [23] to reduce the height and concentration of the carbides by submerging the sample in an electrolytic bath solution with the proper current density used to produce a fine smooth surface polish [24]. The treatment has been cited to reduce the height and concentration of the carbides that is hoped to lead to a reduction in bearing friction between the components. This treatment can
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of Ultra Polish Treated CoCr vs. Standard CoCr surfaces against medical grade standard UHMWPE. A set of six gas plasma sterilized and cleaned compression molded GUR 1050 (non-crosslinked) UHMWPE pin specimens (1.25 in length, 3/8 in dia. tapered bearing tip of 4.2 mm with 301 of tapered angle with Ra¼3339.572666 nm per ASTM 2083) were used. The Ultra Polish Treated and Standard CoCr test disks were approximately 170.02 in diameter with thickness of 3/16 in with surface roughness finish that meets ASTM F732 (ranging from 0.026 to 0.05 mm) commonly used in the orthopedic device industry [27], with a typical carbon content for medical grade CoCr of 0.27 wt%. Tests were conducted under a normal load of 50 N and a frequency of 1 Hz which produced a relative maximum sliding velocity of 35 mm/s against the CoCr counterface with a total traveled distance of 1 km (30,000 cycles) [28]. This compressive load yielded a nominal contact of 3.6 MPa, which lies within the appropriate physiological range for a hip joint (1–3 MPa) [28–31]. The displacement pathway traced an arc of length of 17.4 mm using a multi-directional six station pin-on-disk machine wear tester OrthopodTM (Advanced Mechanical Technology Inc, Waltham, MA). The effect of the polishing procedure on the surface of the CoCr disks was evaluated using non contact profilometry using a WYKO NT2000 profilometer (Veeco Corp., Tucson, AZ). Surface characterization was performed before and after frictional testing at a nominal magnification of 25 (Field of view 736 480 nm, 70.1 nm). All surfaces were characterized for roughness measures arithmetic surface roughness (Ra), root mean squared roughness (Rq), roughness peak mean (Rpm) and roughness valley mean (Rvm). Measurements were taken at 8 points within the locations of expected frictional testing on each disk in a circular fashion in order to fully quantify and characterize the component and ensure reliable and repeatable estimate of surface roughness. All surface roughness analyses were filtered to compensate for macroscopic measures of surface geometry only (tilt and curvature). Statistical analysis (student’s t-test with p¼ 0.05) was performed to evaluate whether there was a significant difference between Standard and Ultra Polish Treated coefficients of friction and surface roughness parameters. Further 2D X–Y tracing was performed on selected areas of interest on both CoCr disks using the surface profilometer to assess discrete peak and valley architectures. To monitor for any unexpected material loss or damage of the UHMWPE pins, their weights were recorded both (Satorius,
be performed on CoCr alloys as they are susceptible to work hardening and finishing processes. Therefore, this surface treatment on the counterface of the polymeric/metal coupling could potentially extend the longevity of the implant. In this study, a short-run reciprocating pin-on-disk friction test was performed between cast CoCr disks (Standard CoCr and Ultra Polish Treated CoCr) slid against standard non-crosslinked ultrahigh molecular weight polyethylene (UHMWPE) pins under standard pin on disk conditions modeled after total joint replacement tribological conditions. This study focused on the evaluation of the CoF for a industry standard material pairing used in THR to assess the effectiveness of this novel surface treatment (Ultra Polish) to not only reduce the incidence of carbide protrusion but also to reduce CoCr-UHMWPE bearing coefficient of friction (CoF).
2. Materials and methods Two reciprocating pin-on-disk friction tests were conducted in series to evaluate and quantify the dynamic friction performance
Dynamic Coefficient of Friction
0.12 0.1 0.08 0.06 0.04
Ultra Polish Treated CoCr Standard CoCr
0.02
Ultra Polish Treated CoCr Standard CoCr
0 0
5000
10000
15000
20000
25000
205
30000
Cycles Fig. 1. Dynamic frictional behavior of Standard and Ultra Polish Treated CoCr disks slid against UHMWPE as a function of cycles traveled (Diluted Bovine Serum at 37 1C).
Table 1 Values of the roughness parameters for both Standard and Ultra Polish Treated CoCr disks before 30,000 cycles of articulations against UHMWPE. (Diluted Bovine Serum at 37 1C). Standard CoCr disks
Mean 7Std. Dev (nm) Max. Min.
Ultra polish CoCr disks
Ra
Rvm
Rpm
Ra
Rvm
Rpm
31.8 78.6 51.7 18.3
364.8 7 199.6 135.9 1162.7
363.9 7 175.9 1114.4 158.9
29.07 8.7 57.1 17.1
722.7 7 295.2 264.9 1412.0
388.5 7 98.4 1114.4 158.9
Table 2 Values of the roughness parameters for both Standard and Ultra Polish Treated CoCr disks after 30,000 cycles of articulations against UHMWPE. (Diluted Bovine Serum at 37 1C). Standard CoCr disks
Mean 7Std. Dev (nm) Max. Min.
Ultra polish CoCr disks
Ra
Rvm
Rpm
Ra
Rvm
Rpm
36 7 13.2 92.80 18.30
376.6 7 203.4 135.90 1162.70
363.9 7175.9 1114.40 158.90
49.6 7 46.4 196.70 16.50
730.77 294.8 264.90 1428.80
387.2 7 97.7 736.90 226.10
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Bohemia, NY, which has an accuracy of 70.0001 g) before and after the 1 km test. Prior to testing, all the samples (CoCr disks, disk holders) were ultrasonically cleaned following ASTM F1715 [32]. During testing, the planar dynamic coefficient of friction for each pin-disk pair was monitored. The coefficient of friction was
p = 0.42
500
Surface Parameters (nm)
300 p = 0.053
100
Rvm Ra
-100
Rpm
-300 -500 Standard CoCr Ultra Polish Treated CoCr
-700 -900 -1100
p < 0.0005*
Fig. 2. Roughness parameters of Standard and Ultra Polish Treated CoCr disks after 30,000 cycles of articulation against UHMWPE. (Diluted Bovine Serum at 37 1C).
collected every 500 repetitions for all six stations with a data acquisition frequency of 200 points/s. Testing was conducted using 25% bovine serum (Hyclone, Logan, UT) and 0.2% sodium azide (NaN3, Fisher Scientific: S227-100) at 37 1C 72 1C following previous studies and according to ASTM F1715-96 [27,33,34]. Sodium azide was used to limit any bacterial growth. The lubricant solution was non circulating in each station and it was kept at 37 1C using a heated water bath around the all the stations. This lubricant was replenished with deionized water as evaporation occurred, and the same lubricant solution was used for the length of the study. At the conclusion of 30,000 cycles of testing, a statistical analysis (student’s t-test with p¼ 0.05) of frictional coefficients for all pins was conducted to determine the effectiveness of the Ultra Polish process to reduce the friction between the polymer/metal couple. At the end of the test, non contact profilometry was performed on both sample groups.
3. Results Fig. 1 presents the six-station means of the dynamic coefficient of friction and their respective standard deviation as a function of sliding cycles for the Ultra Polish Treated and Standard CoCr against UHMWPE. For both groups the initial coefficient of friction is relatively high and then stabilizes over time. The data was fitted to a polynomial trend line of second order, which led to the highest correlation in both cases. No statistical differences were found between the coefficients of friction between material pairs
Fig. 3. Surface profile of an as cast Standard CoCr specimen when slid against non crosslinked UHMWPE under physiological conditions after 30,000 cycles (Diluted Bovine Serum at 37 1C). Significant carbide protrusion can be seen from the alloy surface. (For interpretation of the references to colour in this figure, the reader is referred to the web version of this article.)
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after 30,000 cycles. A power analysis (n¼6, Std. Dev¼0.0137, a ¼0.81) determined that the test enabled accurate and reliable statistical judgments able to detect shifts in the mean down to 0.025. Statistical analysis determined that both the Ultra Polish Treated and Standard disks have a normal distribution of the coefficient of friction after 30,000 cycles (p40.05, Ultra Polish Treated p ¼0.757, Standard CoCr p¼ 0.55). By inspection of both Standard and Ultra Polish Treated disks, there was no visual evidence of a wear track found after the sliding contact of UHMWPE for 1 km. For the CoCr surfaces, no statistical differences in the average surface roughness (Ra) or average maximum profile peaks (Rpm) were found between the Standard and Ultra Polish groups before or after testing, refer to Table 1 and 2. However, maximum profile valley height (Rvm) yielded statistically significant differences (p o0.0005), refer to Table 2. The average maximum profile depth for the Ultra Polish Treated disks was almost doubled the value of the Standard CoCr disks. The Ultra Polish Treatment showed evidence of extracted carbides leaving behind asymmetrical elongated ‘pitted’ regions. A typical representation of the ‘removed’ carbide area is represented on Fig. 6 and the subsequent 2D analysis that shows the typical size in X, Y coordinates of the removed carbide area. Five measurements were taken for three disks to approximate the dimension of the removed carbide area, and found to be of an approximate width of 5.21 71.01 mm and depth of 1.53 70.32 mm. The length was not quantified as it is of an amorphous elongated shape that could possibly incorporate more than one removed carbide area. Fig. 3 presents several surface roughness profiles of the Standard CoCr disks after 30,000 cycles of articulation. These
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profiles show the carbide protrusion (highlighted in dark red and shown with an arrow) distributed randomly across the surface with a maximum average height across all specimens (n¼ 6) measured being 719.9 71003.8 nm for the Standard CoCr disks. Using 2D surface analysis, the heights and widths of five representative CoCr carbide protrusions from each of 3 disks were measured and found to be 11.971.4 mm and 0.1970.01 mm, respectively. (Figs. 2 and 3). Fig. 4 presents a typical carbide feature on the surface of the Standard CoCr disks. The point of interest refers to the highlighted dark red region. This procedure was repeated to quantify the other features on the surface and used to validate the typical height and extension of the carbide colonies, similar to the measures reported in literature [14,16]. The topography of the Ultra Polish Treated disks consists of randomly distributed large and deep pits (highlighted as navy blue bright yellow and shown with an asterisk) seen in Fig. 5. These regions appear to be organized following the grain boundaries of the metal alloy. Other studies have reported similar surface profiles for as cast (ASTM polished) CoCr disks [18]. Maximum average surface heights of the Ultra Polish Treated disks were found to be 545.7 7226.9 nm. Fig. 6 presents a typical pitted area featured on the surface of the Ultra Polish Treated CoCr disks. Five measurements were taken for three Ultra Polish Treated CoCr disks using the 2D analysis to approximate the dimensions of both the width and depth of the pits. The width of the pitted areas were approximately 5.2171.01 mm with a depth of approximately 0.5870.09 mm. The surface topography was also characterized using the SEM as seen in Fig. 7. The carbide colonies are highlighted by the
Fig. 4. (A) Surface profile of a standard CoCr disk articulated against UHMWPE at 30,000 cycles. The cross point represents a selected area of interest it represents the carbide colony protruding through the surface. Typical 2D representative analysis of a selected area of interest (cross point) of an (B) X axis plot showing the width of the area 7 mm and (C) Y axis plot showing the height of the carbide feature 0.3 mm.
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Fig. 5. Representative surface profile of an Ultra Polish Treated CoCr alloy disk, which shows pitted areas as a result of the extraction due to the Ultra Polish procedure. (28.7 ). (For interpretation of the references to colour in this figure, the reader is referred to the web version of this article.)
arrows. These same colonies refer to the highlighted distributed red elongated features in Fig. 3. On the other hand, the removed carbide regions are shown in Fig. 8 for the Ultra Polish Treated CoCr disks. Both low and high magnification photomicrographs illustrate that the removed carbides do not only account for the carbide itself but the Ultra Polish treatment appears to extract the surrounding region as well. The Ultra Polish treatment creates a surface covered with irregular shaped elongated pitted regions. Post polishing or etching of the Ultra Polish Treated surfaces is evident with smoothed pit edges apparent.
4. Discussion The experimental results of this study show that the reported dynamic coefficient of friction for both Standard and Ultra Polish treated disks paired to UHMWPE fall within the values reported in the literature for similar material pairs used in total joint replacements. This study used a similar reciprocating arc pin on disk ASTM standard method used in total joint prosthesis when subjected to physiological loading conditions [35]. The typical traditional reciprocating pin on disk involved the stationary pin with the sliding disk, which created a linear wear path of 50 mm [36]. Due to initial manufacturing requirements as part of the Ultra Polish treatment process, the CoCr disks had small hole in their centers to secure the specimens during polishing. Full scale use of this polishing process on total joint replacement devices would require the manufacture of custom tooling and fixturing during polishing. This study used test parameters that followed ASTM standards: frequency of
1 Hz and maximum sliding velocity of 35 mm/s. The sliding velocity used in this test was within the expected sliding velocity used in reciprocating pin on disk tests for total joint materials (0–50 mm/s as for total joints) [28,37–40]. This study was designed based on previous studies that have used similar test parameters based on the ASTM F732-82 [37]. Saikko et al. developed a three station reciprocating pin on disk test equipment to evaluate the coefficient of friction and wear rates of typical total joint materials such as UHMWPE, CoCr, Al2O3, ZrO2 and Si3N4 under physiological conditions (nominal contact pressure of 4.8 MPa and de-ionized water as lubricant at 1 Hz) [36]. The test conditions involved sliding distance of 50 mm over 7 106 cycles. Previous studies using a reciprocating pin on disk apparatus have reported coefficients of friction between UHMWPE and CoCr under physiological conditions at contact pressures simulating a typical hip replacement (approximately 3 MPa) ranging from 0.03 to 0.15 [29,31,36,41,42]. Quantifying surface roughness parameters allows the possibility not only to evaluate the effect of surface roughness on the friction in a traditional material pairing for total hip joints, but it also enables a better understanding of potential applications for metal on metal total joint replacements. Other studies have only considered the arithmetic mean roughness (Ra) to be directly correlated to the friction when comparing two material pairings. However, in this study there is no significant differentiation between the Ra of Standard and Ultra Polish Treated CoCr disks. This parameter only accounts for the absolute mean values of the measured peaks with relation to the surface, therefore is the main height calculated overall the entire measured length. However, this does not take into account any spatial and textural variation of the topography, which means
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Fig. 6. (A) Surface profile of an Ultra Polish Treated CoCr disk articulated against UHMWPE at 30,000 cycles. The cross point represents a selected area of interest it represents the removed area after the Ultra Polish Treatment. Typical 2D representative analysis of a selected area of interest (cross point) of an (B) X axis plot showing the width of the area 5 mm and (C) Y axis plot showing the depth of the area of interest 0.6 mm.
Fig. 7. Surface microstructure of the Standard CoCr disks after articulation against UHMWPE for 30,000 cycles. (A) 200 , (B) 300 and (C) 300 . The feature highlighted with the arrow is a typical representation of the carbide colony arranged along the grain boundaries protruding towards the surface.
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Fig. 8. Surface microstructure of the Ultra Polish Treated CoCr disks after articulation against UHMWPE for 30,000 cycles. (A) 110 , (B) 230 and (C) 3000 . The feature highlighted with the arrow is a typical representation of the removed carbide region distributed across the entire surface, which is shown in (C) at higher magnification.
that the significant removal of material using the Ultra Polish Treatment does not weigh heavily on this parameter. Therefore, there is need to consider the surface parameters such as Rpm and Rvm that consider the average peaks and average valleys, respectively and represent a more robust value that accounts for the average of several peak heights and valley depths. The Ultra Polish Treated CoCr disks had an average Rvm almost double of what the Standard CoCr disks considering several amorphous shaped pits. The variability in size and depth of these pits is also evidenced in the standard deviation roughness values of the Ultra Polish Treated disks. The material removal was initially hypothesized to be beneficial in order to lower the coefficient of friction. However, these pits create a large surface defect area which could be hypothesized to have a plowing effect that could be a source for abrasive wear and possibly scratching the UHMWPE due to continuous contact. The removal of the hard elastic carbide asperities is presumably intended to reduce the interlocking of asperities that can contribute to increases in friction. Although the carbide removal is evident, it was determined in this study that the process did not result in a significant lowering of the friction coefficient. Rpm represents a better indicator of the degree of pile up since it is the average of the profile peaks through the entire surface and to better characterize the surface finish [43,44]. This pile up is an important parameter that must be considered as it has been shown that even a small increase in this measure (from 0.2 to 2.3 mm) can lead to a significant increase in UHMWPE wear during in vitro studies [45]. Therefore, accounting only for Rp and Rv might not be a representative value as they are sensitive to high peaks and deep valleys and could be a representation of outliers that could not be the true representations of the surface conditions. The effect of the Ultra Polish treatment was intended to reduce the coefficient of friction through the production of a smoother ‘‘carbide free’’ surface. However, this study indicates that there is no significant difference between the coefficients of friction of both groups against UHMWPE. The Ultra Polish treatment was responsible for the removal of not only the carbide but also a
significant zone surrounding the carbide, thus creating a pitted area that could have had an effect on the contact between the articulating components. The lack of statistical difference in the CoF could be also linked to a slight difference in the processing of both tested groups. In the case of the Ultra Polish Treated CoCr, pre-polishing (i.e., hand polishing) was performed before the actual Ultra Polish Treatment. The Standard CoCr disks were ‘polished’ under ASTM F732 standards and did not go through the Ultra Polish Treatment. This difference in ‘pre-treatment’ could have affected the final roughness values, and thus resulting frictional data. However, the pre-polishing would not have been expected to be able to alter the Ultra Polish pitting effect, thus that would remain. A full scale wear test could help determine whether there are significant differences in long term friction and wear between both groups and determine which surface artifact has a larger impact on the coefficient of friction: carbide protrusion in the Standard CoCr disks or the significant pits formed after the Ultra Polish Treatment. The process of carbide removal from metal bearing components could be extremely beneficial for future applications in metal on metal total joints or resurfacing techniques. The removed ‘carbide’ area could serve to trigger an entrapment mechanism for the wear particles as well as a source for continuous lubrication. The ideal lubrication mode in total joint replacements could be considered to be a fluid film regime to promote a load transmitting fluid layer between two solid materials. Possible future studies could include the study of the lubrication regime for Ultra Polish Treated metal on metal joints. Considering that the nature of the lubricating regime is based on the ratio of the thickness of the lubricant film and the surface roughness of the surfaces, the effect of the Ultra Polish Treatment on a metal on metal THR could be significant in maintaining a constant fluid film during lubrication, which would be an significant improvement to the boundary-mixed lubrication regime that is reported in a Metal on Polyethylene THR. However, when
E. Alvarez et al. / Tribology International 47 (2012) 204–211
considering the low wear combination of metal on metal, previous studies do consider that the metallic wear debris and ion release can bring a greater risk to develop hypersensitivity, cytotoxicity and theoretically risk of carcinogenesis for prolonged exposure [46]. It is important to acknowledge that the surface finish of the components can have an effect not only on the friction but also on the damage and wear of the bearing materials. Conversely, if surfaces wear or become damaged, this geometric change can have an reciprocal effect on the lubrication mode. This complex symbiotic relationship between friction, wear and lubrication is best studied under the specific design conditions envisioned for the final bearing assembly and are often modeled computationally. Preliminary measures of surface architecture and coefficients of friction, such as the ones presented in the current work, provide invaluable data to these models for the better design and evaluation of these devices.
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