Tribology International 47 (2012) 204–211

Contents lists available at SciVerse ScienceDirect

Tribology International

0301-67

doi:10.1

n Corr

E-m

journal homepage: www.elsevier.com/locate/triboint

The effect of a novel CoCr electropolishing technique on CoCr-UHMWPEbearing frictional performance for total joint replacements

Alvarez Estefaniaa, Vinciguerra Johnb, DesJardins John Da,n

a Clemson University, Bioengineering Department, 301 Rhodes Research Center, Clemson, SC 29634, USAb DJO Surgical, Austin, TX, USA

a r t i c l e i n f o

Article history:

Received 8 January 2011

Received in revised form

6 June 2011

Accepted 1 November 2011Available online 29 November 2011

Keywords:

Total joint replacement

CoCr

Coefficient of friction

Polishing

9X/$ - see front matter & 2011 Elsevier Ltd.

016/j.triboint.2011.11.002

esponding author.

ail address: jdesjar@clemson.edu (J. DesJardin

a b s t r a c t

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.

1. Introduction

Over the past forty years, the coupling of a cobalt chromemolybdenum (CoCr) metal femoral component with an ultra highmolecular weight polyethylene (UHMWPE) component hasbecome the ‘gold’ standard for a total joint replacement due toits low revision rates and low coefficient of friction [1–5]. Withinthis load bearing couple, CoCr alloys have been considered as thetraditional femoral component material due to its excellentcorrosion resistance, wear resistance, high fatigue strength andits ease for processing. Unfortunately, the accumulation of micronsize UHMWPE wear particles is cited as triggering an adverseosteolytic biological response which eventually leads to asepticloosening of the implant and subsequent significant reduction inthe implant’s longevity [6]. The use of hard scratch resistantcoatings and the use of crosslinked UHMWPE have been shown toimprove the wear performance in this bearing couple [7–13].

Within metal–poly and metal–metal bearing systems, thematerial wear performance of metal alloys has been linked tothe carbon content and heat treatment of the alloy used [14].These two parameters contribute to the carbide formation andeventual protrusion, which is responsible for scratching anddelamination of the UHMWPE [15–18]. CoCr alloys have twophases: cobalt alloy solid solution matrix and metal carbides. Thetwo commercially available CoCr formulations, forged and as cast,

All rights reserved.

s).

mainly differ on the carbon content as well as the distribution andmorphology of the carbides. The forged CoCr (low C contentapproximately 0.067% weight) is known to have a coarse dis-tribution of carbides along the surface. The high carbon cast CoCr(ASTM F75 with approximately 0.26% weight) tends to be proble-matic to control grain size, carbide morphology and distribution[19]. Carbides are known to protrude from the surface of CoCralloys by as much as 0.2 mm [17,18]. They are high hardnesssecond phase materials that are associated not only with accel-erated wear of the softer material of the bearing but also knownto strengthen the alloy matrix [14]. Several studies report that theamount of carbon, carbide volume fraction and carbide morphol-ogy have an effect on the wear resistance on metal on metal THRtested under simulated physiological conditions [14,20–22]. Simi-lar to the current study, these investigations focused on quantify-ing the effect of a novel surface treatment on the surfacemorphology and the resulting frictional behavior between novelmaterial pairings.

To reduce the carbide concentration and protrusion at thesurface of total joint implant surfaces, a surface engineeringtreatment can be used to treat and polish the CoCr femoralcomponents. An example of a surface treatment is the UltraPolish treatment which uses a patented electro polishing treat-ment [23] to reduce the height and concentration of the carbidesby submerging the sample in an electrolytic bath solution withthe proper current density used to produce a fine smooth surfacepolish [24]. The treatment has been cited to reduce the height andconcentration of the carbides that is hoped to lead to a reductionin bearing friction between the components. This treatment can

E. Alvarez et al. / Tribology International 47 (2012) 204–211 205

be performed on CoCr alloys as they are susceptible to workhardening and finishing processes. Therefore, this surface treat-ment on the counterface of the polymeric/metal coupling couldpotentially extend the longevity of the implant.

In this study, a short-run reciprocating pin-on-disk friction testwas performed between cast CoCr disks (Standard CoCr and UltraPolish Treated CoCr) slid against standard non-crosslinked ultra-high molecular weight polyethylene (UHMWPE) pins understandard pin on disk conditions modeled after total joint replace-ment tribological conditions. This study focused on the evaluationof the CoF for a industry standard material pairing used in THR toassess the effectiveness of this novel surface treatment (UltraPolish) to not only reduce the incidence of carbide protrusion butalso to reduce CoCr-UHMWPE bearing coefficient of friction (CoF).

2. Materials and methods

Two reciprocating pin-on-disk friction tests were conducted inseries to evaluate and quantify the dynamic friction performance

0

0.02

0.04

0.06

0.08

0.1

0.12

0

Dyn

amic

Coe

ffic

ient

of

Fri

ctio

n

Cycles

Ultra Polish Treated CoCr

Standard CoCr

Ultra Polish Treated CoCr

Standard CoCr

5000 10000 15000 20000 25000 30000

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 1Values of the roughness parameters for both Standard and Ultra Polish Treated CoCr disk

37 1C).

Standard CoCr disks

Ra Rvm Rpm

Mean7Std. Dev (nm) 31.878.6 �364.87199.6 363.

Max. 51.7 �135.9 1114

Min. 18.3 �1162.7 158.

Table 2Values of the roughness parameters for both Standard and Ultra Polish Treated CoCr dis

37 1C).

Standard CoCr disks

Ra Rvm Rpm

Mean7Std. Dev (nm) 36713.2 �376.67203.4 363.9

Max. 92.80 �135.90 1114

Min. 18.30 �1162.70 158.9

of Ultra Polish Treated CoCr vs. Standard CoCr surfaces againstmedical grade standard UHMWPE. A set of six gas plasma sterilizedand cleaned compression molded GUR 1050 (non-crosslinked)UHMWPE pin specimens (1.25 in length, 3/8 in dia. tapered bearingtip of 4.2 mm with 301 of tapered angle with Ra¼3339.572666 nmper ASTM 2083) were used. The Ultra Polish Treated and StandardCoCr test disks were approximately 170.02 in diameter withthickness of 3/16 in with surface roughness finish that meets ASTMF732 (ranging from 0.026 to 0.05 mm) commonly used in theorthopedic device industry [27], with a typical carbon content formedical grade CoCr of 0.27 wt%. Tests were conducted under anormal load of 50 N and a frequency of 1 Hz which produced arelative maximum sliding velocity of 35 mm/s against the CoCrcounterface 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 oflength of 17.4 mm using a multi-directional six station pin-on-diskmachine wear tester OrthopodTM (Advanced Mechanical TechnologyInc, Waltham, MA).

The effect of the polishing procedure on the surface of the CoCrdisks was evaluated using non contact profilometry using aWYKO NT2000 profilometer (Veeco Corp., Tucson, AZ). Surfacecharacterization was performed before and after frictional testingat a nominal magnification of 25� (Field of view 736�480 nm,70.1 nm). All surfaces were characterized for roughness mea-sures arithmetic surface roughness (Ra), root mean squaredroughness (Rq), roughness peak mean (Rpm) and roughness valleymean (Rvm). Measurements were taken at 8 points within thelocations of expected frictional testing on each disk in a circularfashion in order to fully quantify and characterize the componentand ensure reliable and repeatable estimate of surface roughness.All surface roughness analyses were filtered to compensate formacroscopic measures of surface geometry only (tilt and curva-ture). Statistical analysis (student’s t-test with p¼0.05) wasperformed to evaluate whether there was a significant differencebetween Standard and Ultra Polish Treated coefficients of frictionand surface roughness parameters. Further 2D X–Y tracing wasperformed on selected areas of interest on both CoCr disks usingthe surface profilometer to assess discrete peak and valleyarchitectures.

To monitor for any unexpected material loss or damage of theUHMWPE pins, their weights were recorded both (Satorius,

s before 30,000 cycles of articulations against UHMWPE. (Diluted Bovine Serum at

Ultra polish CoCr disks

Ra Rvm Rpm

97175.9 29.078.7 �722.77295.2 388.5798.4

.4 57.1 �264.9 1114.4

9 17.1 �1412.0 158.9

ks after 30,000 cycles of articulations against UHMWPE. (Diluted Bovine Serum at

Ultra polish CoCr disks

Ra Rvm Rpm

7175.9 49.6746.4 �730.77294.8 387.2797.7

.40 196.70 �264.90 736.90

0 16.50 �1428.80 226.10

E. Alvarez et al. / Tribology International 47 (2012) 204–211206

Bohemia, NY, which has an accuracy of 70.0001 g) before and afterthe 1 km test. Prior to testing, all the samples (CoCr disks, diskholders) were ultrasonically cleaned following ASTM F1715 [32].During testing, the planar dynamic coefficient of friction for eachpin-disk pair was monitored. The coefficient of friction was

-1100

-900

-700

-500

-300

-100

100

300

500

mpRaR

Surf

ace

Par

amet

ers

(nm

)

Standard CoCr

Ultra Polish TreatedCoCr

Rvm

p < 0.0005*

p = 0.053

p = 0.42

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).

Fig. 3. Surface profile of an as cast Standard CoCr specimen when slid against non crossl

Serum at 37 1C). Significant carbide protrusion can be seen from the alloy surface. (For i

web version of this article.)

collected every 500 repetitions for all six stations with a dataacquisition frequency of 200 points/s. Testing was conductedusing 25% bovine serum (Hyclone, Logan, UT) and 0.2% sodiumazide (NaN3, Fisher Scientific: S227-100) at 37 1C72 1C followingprevious studies and according to ASTM F1715-96 [27,33,34].Sodium azide was used to limit any bacterial growth. Thelubricant solution was non circulating in each station and it waskept at 37 1C using a heated water bath around the all thestations. This lubricant was replenished with deionized water asevaporation occurred, and the same lubricant solution was usedfor the length of the study. At the conclusion of 30,000 cycles oftesting, a statistical analysis (student’s t-test with p¼0.05) offrictional coefficients for all pins was conducted to determine theeffectiveness of the Ultra Polish process to reduce the frictionbetween the polymer/metal couple. At the end of the test, noncontact profilometry was performed on both sample groups.

3. Results

Fig. 1 presents the six-station means of the dynamic coefficientof friction and their respective standard deviation as a function ofsliding cycles for the Ultra Polish Treated and Standard CoCragainst UHMWPE. For both groups the initial coefficient of frictionis relatively high and then stabilizes over time. The data was fittedto a polynomial trend line of second order, which led to thehighest correlation in both cases. No statistical differences werefound between the coefficients of friction between material pairs

inked UHMWPE under physiological conditions after 30,000 cycles (Diluted Bovine

nterpretation of the references to colour in this figure, the reader is referred to the

E. Alvarez et al. / Tribology International 47 (2012) 204–211 207

after 30,000 cycles. A power analysis (n¼6, Std. Dev¼0.0137,a¼0.81) determined that the test enabled accurate and reliablestatistical judgments able to detect shifts in the mean down to0.025. Statistical analysis determined that both the Ultra PolishTreated and Standard disks have a normal distribution of thecoefficient of friction after 30,000 cycles (p40.05, Ultra PolishTreated p¼0.757, Standard CoCr p¼0.55).

By inspection of both Standard and Ultra Polish Treated disks,there was no visual evidence of a wear track found after thesliding contact of UHMWPE for 1 km. For the CoCr surfaces, nostatistical differences in the average surface roughness (Ra) oraverage maximum profile peaks (Rpm) were found between theStandard and Ultra Polish groups before or after testing, refer toTable 1 and 2. However, maximum profile valley height (Rvm)yielded statistically significant differences (po0.0005), refer toTable 2. The average maximum profile depth for the Ultra PolishTreated disks was almost doubled the value of the Standard CoCrdisks. The Ultra Polish Treatment showed evidence of extractedcarbides leaving behind asymmetrical elongated ‘pitted’ regions.A typical representation of the ‘removed’ carbide area is repre-sented on Fig. 6 and the subsequent 2D analysis that shows thetypical size in X, Y coordinates of the removed carbide area. Fivemeasurements were taken for three disks to approximate thedimension of the removed carbide area, and found to be ofan approximate width of 5.2171.01 mm and depth of1.5370.32 mm. The length was not quantified as it is of anamorphous elongated shape that could possibly incorporate morethan one removed carbide area.

Fig. 3 presents several surface roughness profiles of theStandard CoCr disks after 30,000 cycles of articulation. These

Fig. 4. (A) Surface profile of a standard CoCr disk articulated against UHMWPE at 30,0

carbide colony protruding through the surface. Typical 2D representative analysis of a s

area �7 mm and (C) Y axis plot showing the height of the carbide feature �0.3 mm.

profiles show the carbide protrusion (highlighted in dark red andshown with an arrow) distributed randomly across the surfacewith a maximum average height across all specimens (n¼6)measured being 719.971003.8 nm for the Standard CoCr disks.Using 2D surface analysis, the heights and widths of five repre-sentative CoCr carbide protrusions from each of 3 disks weremeasured 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 theStandard CoCr disks. The point of interest refers to the highlighteddark red region. This procedure was repeated to quantify theother features on the surface and used to validate the typicalheight and extension of the carbide colonies, similar to themeasures reported in literature [14,16].

The topography of the Ultra Polish Treated disks consists ofrandomly distributed large and deep pits (highlighted as navyblue bright yellow and shown with an asterisk) seen in Fig. 5.These regions appear to be organized following the grain bound-aries of the metal alloy. Other studies have reported similarsurface profiles for as cast (ASTM polished) CoCr disks [18].Maximum average surface heights of the Ultra Polish Treateddisks were found to be 545.77226.9 nm. Fig. 6 presents a typicalpitted area featured on the surface of the Ultra Polish TreatedCoCr disks. Five measurements were taken for three Ultra PolishTreated CoCr disks using the 2D analysis to approximate thedimensions of both the width and depth of the pits. The width ofthe pitted areas were approximately 5.2171.01 mm with a depthof approximately 0.5870.09 mm.

The surface topography was also characterized using the SEMas seen in Fig. 7. The carbide colonies are highlighted by the

00 cycles. The cross point represents a selected area of interest it represents the

elected area of interest (cross point) of an (B) X axis plot showing the width of the

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.)

E. Alvarez et al. / Tribology International 47 (2012) 204–211208

arrows. These same colonies refer to the highlighted distributedred elongated features in Fig. 3. On the other hand, the removedcarbide regions are shown in Fig. 8 for the Ultra Polish TreatedCoCr disks. Both low and high magnification photomicrographsillustrate that the removed carbides do not only account for thecarbide itself but the Ultra Polish treatment appears to extract thesurrounding region as well. The Ultra Polish treatment creates asurface covered with irregular shaped elongated pitted regions.Post polishing or etching of the Ultra Polish Treated surfaces isevident with smoothed pit edges apparent.

4. Discussion

The experimental results of this study show that the reporteddynamic coefficient of friction for both Standard and Ultra Polishtreated disks paired to UHMWPE fall within the values reportedin the literature for similar material pairs used in total jointreplacements. This study used a similar reciprocating arc pin ondisk ASTM standard method used in total joint prosthesis whensubjected to physiological loading conditions [35]. The typicaltraditional reciprocating pin on disk involved the stationary pinwith the sliding disk, which created a linear wear path of 50 mm [36].Due to initial manufacturing requirements as part of the Ultra Polishtreatment process, the CoCr disks had small hole in their centers tosecure the specimens during polishing. Full scale use of this polishingprocess on total joint replacement devices would require the man-ufacture of custom tooling and fixturing during polishing. This studyused test parameters that followed ASTM standards: frequency of

1 Hz and maximum sliding velocity of 35 mm/s. The sliding velocityused in this test was within the expected sliding velocity used inreciprocating pin on disk tests for total joint materials (0–50 mm/s asfor total joints) [28,37–40]. This study was designed based onprevious studies that have used similar test parameters based onthe ASTM F732-82 [37]. Saikko et al. developed a three stationreciprocating pin on disk test equipment to evaluate the coefficient offriction and wear rates of typical total joint materials such asUHMWPE, CoCr, Al2O3, ZrO2 and Si3N4 under physiological conditions(nominal contact pressure of 4.8 MPa and de-ionized water aslubricant at 1 Hz) [36]. The test conditions involved sliding distanceof 50 mm over 7�106 cycles. Previous studies using a reciprocatingpin on disk apparatus have reported coefficients of friction betweenUHMWPE and CoCr under physiological conditions at contact pres-sures 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 possibilitynot only to evaluate the effect of surface roughness on the friction in atraditional material pairing for total hip joints, but it also enables abetter understanding of potential applications for metal on metaltotal joint replacements. Other studies have only considered thearithmetic mean roughness (Ra) to be directly correlated to thefriction when comparing two material pairings. However, in thisstudy there is no significant differentiation between the Ra ofStandard and Ultra Polish Treated CoCr disks. This parameter onlyaccounts for the absolute mean values of the measured peaks withrelation to the surface, therefore is the main height calculated overallthe entire measured length. However, this does not take into accountany spatial and textural variation of the topography, which means

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.

E. Alvarez et al. / Tribology International 47 (2012) 204–211 209

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.

E. Alvarez et al. / Tribology International 47 (2012) 204–211210

that the significant removal of material using the Ultra PolishTreatment does not weigh heavily on this parameter. Therefore, thereis need to consider the surface parameters such as Rpm and Rvm thatconsider the average peaks and average valleys, respectively andrepresent a more robust value that accounts for the average of severalpeak heights and valley depths. The Ultra Polish Treated CoCr diskshad an average Rvm almost double of what the Standard CoCr disksconsidering several amorphous shaped pits. The variability in size anddepth of these pits is also evidenced in the standard deviationroughness values of the Ultra Polish Treated disks. The materialremoval was initially hypothesized to be beneficial in order to lowerthe coefficient of friction. However, these pits create a large surfacedefect area which could be hypothesized to have a plowing effect thatcould be a source for abrasive wear and possibly scratching theUHMWPE due to continuous contact. The removal of the hard elasticcarbide asperities is presumably intended to reduce the interlockingof asperities that can contribute to increases in friction. Although thecarbide removal is evident, it was determined in this study that theprocess did not result in a significant lowering of the frictioncoefficient. Rpm represents a better indicator of the degree of pileup since it is the average of the profile peaks through the entiresurface and to better characterize the surface finish [43,44]. This pileup is an important parameter that must be considered as it has beenshown that even a small increase in this measure (from 0.2 to2.3 mm) can lead to a significant increase in UHMWPE wear duringin vitro studies [45]. Therefore, accounting only for Rp and Rv mightnot be a representative value as they are sensitive to high peaks anddeep valleys and could be a representation of outliers that could notbe the true representations of the surface conditions.

The effect of the Ultra Polish treatment was intended to reducethe coefficient of friction through the production of a smoother‘‘carbide free’’ surface. However, this study indicates that there isno significant difference between the coefficients of friction ofboth groups against UHMWPE. The Ultra Polish treatment wasresponsible for the removal of not only the carbide but also a

significant zone surrounding the carbide, thus creating a pittedarea that could have had an effect on the contact between thearticulating components. The lack of statistical difference in theCoF could be also linked to a slight difference in the processing ofboth tested groups. In the case of the Ultra Polish Treated CoCr,pre-polishing (i.e., hand polishing) was performed before theactual Ultra Polish Treatment. The Standard CoCr disks were‘polished’ under ASTM F732 standards and did not go throughthe Ultra Polish Treatment. This difference in ‘pre-treatment’could have affected the final roughness values, and thus resultingfrictional data. However, the pre-polishing would not have beenexpected to be able to alter the Ultra Polish pitting effect, thusthat would remain. A full scale wear test could help determinewhether there are significant differences in long term friction andwear between both groups and determine which surface artifacthas a larger impact on the coefficient of friction: carbide protru-sion in the Standard CoCr disks or the significant pits formed afterthe Ultra Polish Treatment.

The process of carbide removal from metal bearing compo-nents could be extremely beneficial for future applications inmetal on metal total joints or resurfacing techniques. Theremoved ‘carbide’ area could serve to trigger an entrapmentmechanism for the wear particles as well as a source forcontinuous lubrication. The ideal lubrication mode in total jointreplacements could be considered to be a fluid film regime topromote a load transmitting fluid layer between two solidmaterials. Possible future studies could include the study of thelubrication regime for Ultra Polish Treated metal on metal joints.Considering that the nature of the lubricating regime is based onthe ratio of the thickness of the lubricant film and the surfaceroughness of the surfaces, the effect of the Ultra Polish Treatmenton a metal on metal THR could be significant in maintaining aconstant fluid film during lubrication, which would be an sig-nificant improvement to the boundary-mixed lubrication regimethat is reported in a Metal on Polyethylene THR. However, when

E. Alvarez et al. / Tribology International 47 (2012) 204–211 211

considering the low wear combination of metal on metal, pre-vious studies do consider that the metallic wear debris and ionrelease can bring a greater risk to develop hypersensitivity,cytotoxicity and theoretically risk of carcinogenesis for prolongedexposure [46]. It is important to acknowledge that the surfacefinish of the components can have an effect not only on thefriction but also on the damage and wear of the bearing materials.Conversely, if surfaces wear or become damaged, this geometricchange can have an reciprocal effect on the lubrication mode. Thiscomplex symbiotic relationship between friction, wear and lubri-cation is best studied under the specific design conditions envi-sioned for the final bearing assembly and are often modeledcomputationally. Preliminary measures of surface architectureand coefficients of friction, such as the ones presented in thecurrent work, provide invaluable data to these models for thebetter design and evaluation of these devices.

Acknowledgments

These studies were funded by DJO Surgical.

References

[1] Goodman S, Wright T. 2007 AAOS/NIH Osteolysis and implant wear:biological, biomedical engineering, and surgical principles. introduction.Journal of the American Academy of Orthopaedic Surgeons 2008;16(1):x–xi.

[2] Malchau H, Herberts P, Ahnfelt L. Prognosis of total hip-replacement inSweden - Follow-up of 92,675 operations performed 1978–1990. ActaOrthopaedica Scandinavica 1993;64(5):497–506.

[3] Mitchell JC, Shardlow DL, Mohan R, Stone MH. Is the Charnley Hip Still thegold standard? the medium-term results of the charnley implanted through aposterior approach Journal of Bone and Joint Surgery 2005;87-B(Suppl-I):42.

[4] Sibanda N, Copley LP, Lewsey JD, Borroff M, Gregg P, MacGregor AJ, PickfordM, Porter M, Tucker K, van der Meulen JH, on behalf of the SteeringCommittee of the National Joint Registry for England and Wales. Revisionrates after primary hip and knee replacement in England between 2003 and2006. PLoS Medicine 2008;5(9):e179.

[5] Dowson D. New joints for the millennium: wear control in total replacementhip joints. Proceedings of the Institution of Mechanical Engineers, Part H:Journal of Engineering in Medicine 2001;215(4):335–58.

[6] Ingham E, Fisher J. Biological reactions to wear debris in total joint replace-ment. Proceedings of the Institution of Mechanical Engineers Part H-Journalof Engineering in Medicine 2000;214(H1):21–37.

[7] Vaidya, C, Alvarez, E, Vinciguerra, J, Bruce, D.A, and DesJardins, JD, Reductionin Total Knee Replacement Wear with Vitamin E Doped Highly CrosslinkedUhmwpe, Proceedings of the Institution of Mechanical Engineers, Part H:Journal of Engineering in Medicine, [In Publication] (2010).

[8] Oral E, Beckos CAG, Lozynsky AJ, Malhi AS, Muratoglu OK. Improvedresistance to wear and fatigue fracture in high pressure crystallized vitaminE-containing ultra-high molecular weight polyethylene. Biomaterials2009;30(10):1870–80.

[9] Shanbhag A, Rubash H, Jacobs J. Joint Replacement and Bone Resorption. NewYork: Taylor and Francis; 2006 pp. 1–783.

[10] Dearnaley G, Arps JH. Biomedical applications of diamond-like carbon (Dlc)coatings: a review. Surface and Coatings Technology 2005;200(7):2518–24.

[11] Oral E, Wannomae KK, Hawkins N, Harris WH, Muratoglu OK. Alpha-Tocopherol-doped irradiated uhmwpe for high fatigue resistance and lowwear. Biomaterials 2004;25(24):5515–22.

[12] Ezzet KA, Hermida JC, Colwell CW, D’Lima DD. Oxidized zirconium femoralcomponents reduce polyethylene wear in a knee wear simulator. ClinicalOrthopaedics and Related Research 2004;428:120–4.

[13] Fisher J, Hu XQ, Stewart TD, Williams S, Tipper JL, Ingham E, Stone MH,Davies C, Hatto P, Bolton J, Riley M, Hardaker C, Isaac GH, Berry G. Wear ofsurface engineered metal-on-metal hip prostheses. Journal of MaterialsScience: Materials in Medicine 2004;15(3):225–35.

[14] Varano R, Bobyn J, Medley J, Yue S. The effect of microstructure on the wearof cobalt-based alloys used in metal-on-metal hip implants. Proceedings ofthe Institution of Mechanical Engineers, Part H: Journal of Engineering inMedicine 2006;220(2):145–59.

[15] Harman MK, DesJardins J, Benson L, Banks SA, LaBerge M, Hodge WA.Comparison of polyethylene tibial insert damage from in vivo functionand in vitro wear simulation. Journal of Orthopaedic Research 2009;27(4):540–8.

[16] Klepper CC, Williams JM, Truhan JJ, Qu J, Riester L, Hazelton RC, Moschella JJ,Blau PJ, Anderson JP, Popoola OO, Keitz MD. Tribo-mechanical properties ofthin boron coatings deposited on polished cobalt alloy surfaces for orthope-dic applications. Thin Solid Films 2008;516(10):3070–80.

[17] Williams JM, Riester L, Pandey R, Eberhardt AW. Properties of nitrogen-implanted alloys and comparison materials. Surface and Coatings Technology1997;88(1–3):132–8.

[18] Que L, Topoleski LDT, Parks NL. Surface roughness of retrieved cocrmo alloyfemoral components from pca artificial total knee joints. Journal of Biome-dical Materials Research 2000;53(1):111–8.

[19] Chiba A, Kumagai K, Nomura N, Miyakawa S. Pin-on-disk wear behavior in alike-on-like configuration in a biological environment of high carbon cast andlow carbon forged Co-29cr-6mo alloys. Acta Materialia 2007;55(4):1309–18.

[20] Cawley, J, Metcalf, JEP, Jones, AH, Band, TJ, and Skupien, DS, A TribologicalStudy of Cobalt Chromium Molybdenum Alloys Used in Metal-on-MetalResurfacing Hip Arthroplasty, Wear, 255 [7–12] p. 999–1006.

[21] John St KR, Zardiackas LD, Poggie RA. Wear Evaluation of cobalt-chromiumalloy for use in a metal-on-metal hip prosthesis. J. Biomed. Mater. Res. Part B2004;68B(1):1–14.

[22] Tipper JL, Firkins PJ, Ingham E, Fisher J, Stone MH, Farrar R. Quantitativeanalysis of the wear and wear debris from low and high carbon contentcobalt chrome alloys used in metal on metal total hip replacements. Journalof Materials Science: Materials in Medicine 1999;10(6):353–62.

[23] Roger, GJ, Surface Finishing of Cocrmo for Knee Implants, ISTA Conference2002, (2002).

[24] Roger, G, James, Surface Preparation of an Implant, St.Leonards Pat. 26.08.2004.[27] Standard Test Method for Wear Testing of Polymeric Materials Used in Total

Joint Prostheses, Astm Designation F 732. American Society for Testing andMaterials, Philadelphia, Pa.

[28] Chawan, AD, Chakravartula, AM, Zhou, J, Pruitt, LA, Ries, M, and Komvopou-los, K, Effects of Counterface Roughness and Conformity on the TribologicalPerformance of Crosslinked and Non-Crosslinked Medical Grade Ultra HighMolecular Weight Polyethylene, pp. N5.10.1-N5.6 in vol. 724, MaterialsResearch Society (Conference Proceedings Mrs). 2002.

[29] Mazzucco D, Spector M. Contact Area as a critical determinant in thetribology of metal-on-polyethylene total joint arthroplasty. Journal of Tribol-ogy-Transactions of the Asme 2006;128(1):113–21.

[30] Roy Chowdhury SK, Kulkarni AC, Basak A, Roy SK. Wear characteristic andbiocompatibility of some hydroxyapatite-collagen composite acetabularcups. Wear 2007;262(11–12):1387–98.

[31] Saikko V. Effect of contact pressure on wear and friction of ultra-highmolecular weight polyethylene in multidirectional sliding. Proceedings ofthe Institution of Mechanical Engineers Part H-Journal of Engineering inMedicine 2006;220(H7):723–31.

[32] Standard Guide for Wear Assesment of Prosthetic Knee Designs in SimulatorDevices, Astm Designation F 1715. American Society for Testing and Materi-als, Philadelphia, Pa.

[33] DesJardins J, Aurora A, Tanner SL, Pace TB, Acampora KB, LaBerge M.Increased total knee arthroplasty ultra-high molecular weight polyethylenewear using a clinically relevant hyaluronic acid simulator lubricant. Proceed-ings of the Institution of Mechanical Engineers Part H-Journal of Engineeringin Medicine 2006;220(H5):609–23.

[34] McKellop H, Clarke IC, Markolf KL, Amstutz HC. Wear characteristics ofUhmw Polyethylene: a method for accurately measuring extremely low wearrates. Journal of Biomedical Materials Research 1978;12(6):895–927.

[35] Turell M, Wang AG, Bellare A. Quantification of the effect of cross-pathmotion on the wear rate of ultra-high molecular weight polyethylene. Wear2003;255:1034–9.

[36] Saikko V. Wear and friction properties of prosthetic joint materials evaluatedon a reciprocating pin-on-flat apparatus. Wear 1993;166(2):169–78.

[37] Evaluation of Friction and Wear Properties of Polymeric Materials for Use inTotal Joint Prostheses Using Micro-Tribometer Mod., Astm Designation F732–82. American Society for Testing and Materials, Philadelphia, Pa.

[38] Fisher J, Dowson D, Hamdzah H, Lee HL. The effect of sliding velocity on thefriction and wear of Uhmwpe for use in total artificial joints. Wear1994;175(1–2):219–25.

[39] Gevaert MR, LaBerge M, Gordon JM, DesJardins JD. The quantification ofphysiologically relevant cross-shear wear phenomena on orthopedic bearingmaterials using the max-shear wear testing system. Journal of Tribology-Transactions of the Asme 2005;127(4):740–9.

[40] Klapperich C, Komvopoulos K, Pruitt L. Tribological properties and micro-structure evolution of ultra-high molecular weight polyethylene. Journal ofTribology-Transactions of the Asme 1999;121(2):394–402.

[41] Chowdhury SKR, Mishra A, Pradhan B, Saha D. Wear characteristic andbiocompatibility of some polymer composite acetabular cups. Wear2004;256(11–12):1026–36.

[42] Chawan, AD, Chakravartula, AM, Zhou, J, Pruitt, LA, Ries, M, and Komvopou-los, K, in Materials Research Society, vol. 724, p. N5.10.1–6. MaterialsResearch Symposium Proceedings, 2002.

[43] Levesque, M., Livingston, B., Jones, W., and Spector, M., in: 44th AnnualMeeting Orthopaedic Research Society, New Orleans, Lousiana, 1998.

[44] Lancaster JG, Dowson D, Isaac GH, Fisher J. The wear of ultra-high molecularweight polyethylene sliding on metallic and ceramic counterfaces represen-tative of current femoral surfaces in joint replacement. Proceedings of theInstitution of Mechanical Engineers Part H-Journal of Engineering in Medi-cine 1997;211(1):17–24.

[45] Dowson D, Taheri S, Wallbridge NC. The role of counterface imperfections inthe wear of polyethylene. Wear 1987;119(3):277–93.

[46] Tipper JL, Ingham E, Jin ZM, Fisher J. The science of metal-on-metalarticulation. Current Orthopaedics 2005;19(4):280–7.