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Wear 267 (2009) 683–688

Contents lists available at ScienceDirect

Wear

journa l homepage: www.e lsev ier .com/ locate /wear

ffect of metallic nanoparticles on the biotribocorrosion behaviour ofetal-on-Metal hip prostheses

. Yana,∗, A. Nevillea, D. Dowsona, S. Williamsb, J. Fisherb

Institute of Engineering Thermofluids, Surfaces and Interfaces (iETSI), School of Mechanical Engineering, University of Leeds, Leeds, West Yorkshire LS2 9JT, UKInstitute of Medical and Biological Engineering (iMBE), School of Mechanical Engineering, University of Leeds, Leeds LS2 9J, UK

r t i c l e i n f o

rticle history:eceived 15 September 2008eceived in revised form 10 December 2008ccepted 10 December 2008

eywords:ribocorrosionebrisear

a b s t r a c t

Concerns of polyethylene wear debris induced osteolysis has been partially responsible for the renewedinterest in the second generation Metal-on-Metal (MoM) hip replacements as an alternative to widelyused Metal-on-Polyethylene (MoP) joint prostheses. The total wear loss from MoM is much less thanfrom MoP. However, the number of nano-sized wear particles from MoM is more than a hundredtimes than from MoP. Some of the particles are released to the surrounding tissues and some remainin the bearing contact area as a third-body. This paper focuses on the third-body particles and theireffect on tribology and corrosion processes to themselves and the bearing surfaces. A friction hipsimulator was used, integrated with an electrochemical cell to study the biotribocorrosion system. Pre-

ip prosthesesribologyorrosion

fabricated cobalt particles (28 nm in diam.) were employed. The diameter of all femoral heads involvedin this study was 36 mm. The open circuit potential (OCP) was monitored with and without thesenano-size Co particles. After each test, particles were isolated and observed via TEM (Transmission Elec-tron Microscopy). ICP-MS (Inductively Coupled Plasma Mass Spectroscopy) was used to quantify thereleased metal ions. The friction factor and the rate of metal ion release were influenced by the addednanoparticles and in this paper the role of nanoparticles in the biotribocorrosion process is discussed.

. Introduction

The introduction of joint arthroplasty provides a surgical solu-ion for patients to relieve pain and restore the mobility of theoint [1]. However, loosening of the prosthesis is recognized ashe most common cause of failure of total hip and other arthro-lasties [2]. Adverse tissue reactions to prosthetic wear particlesre believed to be an important factor in the development ofsteolysis and a cause of loosening, especially from Metal-on-olyethylene (MoP) type of joint implants [3]. Due to osteolysisnd loosening, revision of the prosthesis is normally needed afternly about 10 years or even sooner for some cases [4]. Because ofhe demand of joint replacements (total and resurfacing) in morective and younger patients with life expectancies of these devices

fter surgery in excess of 25 years, hard-on-hard hip implantsave been recommended. This category includes Metal-on-MetalMoM), Ceramic-on-Metal (CoM) and Ceramic-on-Ceramic (CoC)ypes of hip replacements, which differ from hard-on-soft typesuch as MoP one. Many factors affect the longevity of the service

∗ Corresponding author. Tel.: +44 1133432106; fax: +44 1132424611.E-mail address: Y.Yan@leeds.ac.uk (Y. Yan).

043-1648/$ – see front matter © 2009 Elsevier B.V. All rights reserved.oi:10.1016/j.wear.2008.12.110

© 2009 Elsevier B.V. All rights reserved.

time of those devices, such as the materials, design, manufacturingparameters, etc. Research has proved that by using a larger diam-eter of femoral hip head (>28 mm) and an optimized clearance,reduced total wear (material degradation) can be achieved throughthe presence of an iso-viscoelastic elastohydrodynamic lubrication[5]. However entrapped wear debris has been recognized as one ofthe main caused of scratching of the metallic bearing surfaces [6].It acts like an abrasive and results in an enhancement of materialremoval. It is defined as the third-body with the two relatively mov-ing surfaces as the first and the second body. Studies have shownthat the amount and the size of the abrasives can affect the wearrate [7,8].

A lower wear rate is critical for extending implant life. Materialloss from MoM articulations has been estimated to be 40–100 timeslower than MoP combinations [9]. It has been studied that the mostactive (causing osteolysis) particles are in a size range of 0.1–10 �mfor polyethylene particles [10]. For metal hip replacements, weardebris was isolated from previously implanted prosthetic tissuesand also from simulators. It has been shown that the mean size

of 20–80 nm particles is significantly too small to affect the resi-dent macrophages and below the activation of the osteolytic sizerange [11]. Therefore, although the total volumetric wear loss islower for MoM than for MoP, the number of wear particles isgreater by about a factor of 2. The long-term effect of nano-sized

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etallic debris and resulting metallic ions are still not fully under-tood.

Material degradation in metallic joint implants was thought toe solely mechanical induced from tribological movements and

oading. However, these devices operate in a biological and corro-ive environment. The material loss has been shown to be due touch more complicated processes [12–17]. Materials used in sur-

ical implants, normally have very high corrosion resistance. It isue to the passive film on the material surface, which acts as a bar-ier to the release of ions. However, this film can be damaged byhe relative motion upon the surface resulting in an accelerationf corrosion processes [14–17]. Biotribocorrosion is the study tonderstand the interactions between tribology (mechanical wear)nd electrochemical (corrosion) processes in biological environ-ents and is the focus of this paper.Cobalt chromium molybdenum (CoCrMo) alloys are used widely

n load bearing implants due to their superior mechanical proper-ies to resist wear. They have two phases consisting a solid solution

atrix and metal carbides. The spontaneously formed passive films a mixture of chromium and cobalt oxides in nature. The film isusceptible to fracture owing to mechanical loading resulting incratches and denting [18]. It can ‘repair’ itself rapidly after beingamaged. The process is called repassivation. The process of theemoval of the passive film is defined as depassivation.

Studies on the biotribocorrosion behaviour for CoCrMo alloyave been previously published [13–17]. However, the understand-

ng of the third-body effect on the biotribocorrosion system is stillar from complete. In the paper, integrated tribo-electrochemical

easurements have been carried out in a friction simulator andome contributions are made to the processes occurring at MoMnterfaces.

. Experimental methods

A pendulum friction simulator was used with the integratedlectrochemical cell to enable the study of the biotribocorro-ion system (Fig. 1). It consists of a single-station servo-hydraulicachine, controlled by a PC via a graphic user interface, which

an apply a dynamic loading cycle. A fixed frame mounted on tworessurized hydrostatic bearings formed the friction carriage. Aiezoelectric transducer connected to the front of friction carriageetermined the frictional torque in the system. The acetabular cupas located in the cup holder, mounted in the centre of the friction

arriage. The femoral head was attached to the loading frame viahe motion arm [19]. A Ag/AgCl reference electrode and a platinumounter electrode were fixed with the cup holder which enableshe tribo-corrosion tests. All hips tested were 36 mm in diameternd made from wrought CoCrMo alloys (DePuy International Ltd.,

K). Bovine serum (25%, v/v) and 0.3% NaCl saline solution weremployed as lubricants in this study. The volume of lubricants usedor each test was 5 ml.

The bearing surfaces were isolated from the main frame tovoid any interferences. The open circuit potential (OCP) was mon-

Fig. 1. Schematic diagram for the integrated friction simulator.

Fig. 2. Two loading conditions: (a) 300 N constant load and (b) 2 kN peak load with300 N swing load.

itored while implants were subjected to loading cycles and wereperformed for every test. In an electrolyte, the potential that thematerial adopts when it is not connected to any external electricalsource is OCP. At OCP the anodic reaction rate is equal to the cathodicreaction rate. The OCP can give a semi-quantitative assessment ofthe corrosion regime in which the material resides. Materials wereexposed to tribological contact; the passive film of the tested spec-imens was removed and re-built. The OCP measurement can give afirst in situ indication of this process.

The open circuit potential (OCP) was monitored while implantswere under going the following loading cycles (1 Hz):

• flexion–extension of ±25◦ applied to the head and a constant loadof 300 N (Fig. 2(a))

• flexion–extension of ±25◦ applied to the head and the appliedas a simple sinusoidal waveform over 60% of each loading cycle,with a peak of 2 kN, and swing phase load of 300 N (2 kN 300 N)(Fig. 2(b))

ICP-MS (Inductively Coupled Plasma Mass Spectroscopy) mea-surements were conducted to analyze the released metallic ionsinto the solution. TEM was used to characterize the nanoparticles.Cobalt particles used in this study were approximately 28 nm indiameter. The size distribution of the as-received particles is shownin Fig. 3(a) and a microscopic image taken from TEM of the CoCrparticles is shown in Fig. 3(b). The amount of particles added inthe head–cup contact was up to 50 mg, which is the equivalent to6 years of debris generated from a real hip implant if all debrisremains in the hip capsule. Additionally, 50 mg Cobalt particleswere immersed in 0.3% NaCl and serum (25%, v/v) at 37 ◦C with notribological contact and the liquid solutions were examined by ICPto investigate the dissolution rate under static conditions. Samplesfor ICP tests were prepared in the following way. One ml super-natant sample after being centrifuged for 5 min was added with1 ml 5% nitric acid, followed by 10 min 65 ◦C water bath. The sam-ple was then centrifuged again. One ml supernatant was collected,than diluted with 1.5 ml deionized water.

The head–cup couple was soaked for 4 min to enable the for-

mation of a passive film before the load and motion were applied.The dynamic test was conducted for 20 min. Different weightof cobalt particles (1 mg, 5 mg, 10 mg, 30 mg and 50 mg) wereadded every 3 min in the system at 300 N constant load withthe flexion–extension angle of ±25◦ applied to the head to studywhether a critical particle content for depassivation exists.

Y. Yan et al. / Wear 267 (2009) 683–688 685

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that particles were adhered to and rubbed off from the head andcup surfaces, which resulted in an unstable OCP. TEM images areshown in Fig. 7. The flake-shaped debris may be from the deforma-tion process under loading. In addition, agglomeration of particleswas observed. Very small debris was seen under high load. It seems

Fig. 3. Artificial cobalt particles: (a)

. Results and discussion

Implants were immersed in saline solutions to enable a sta-le formation of a passive layer. As soon as the load was addednd the motion started, a dramatic drop of the OCP was observedFig. 4). It indicates that the protective film was locally removed. TheCP value stabilized after the initial period, which was induced by

he balance between mechanical depassivation and electrochemi-al repassivation. The tribofilm formed on the surface was no longersimple passive film (oxides). Previous studies have demonstrated

hat for CoCrMo alloys, in saline solution, a thickened film con-isting chromium/cobalt oxides and hydroxides was found underribological contact [15]. In a protein containing environment, mix-ures of organometallic and metallic oxides were observed in theormation of the tribofilm [16]. When 50 mg nanoparticles weredded, a shift of potential to a more negative direction was seenFig. 5). The rate of the OCP decrease was slower than the situa-ion without added particles. It may be because with small particlesnvolved in the contact, the damage on the tribofilm was more local-zed. The rate of the change of the OCP indicates the severity of theamage on the tribofilm [12]. Comparing two loading conditions, atigher load (2 kN, 300 N), the OCP shifted to a more negative valueFig. 5(b)).

Optical microscopic images for the head surfaces with and with-ut the third-body particles in the saline solution after testing arehown in Fig. 6. Clear scratches and pits were seen on the sur-ace with no additional particles (Fig. 6(a)). Scratches were inducedy the combination of the two-body abrasion and the third-bodybrasion mechanisms. It means that due to different surface pro-les (surface roughness, hardness, etc.) when one component, inhis study the femoral head, moves relative to its counterpart (theup), material was removed and wear debris was generated. After

he generation of the small particles, the third-body abrasive wearas the predominant wear mechanism. By inserting artificial nano-obalt particles, the intention was to accelerate the wear processes.

n Fig. 6(b), under 300 N constant load, besides the obvious wearrooves, trapped dark particles can also be observed. At high loading

ig. 4. The OCP and the friction factor as a function of time for the hip implant in.3% NaCl under 300 N.

stribution chart and (b) TEM image.

condition with 2 kN 300 N, transferred material from the parti-cle to the head surface indicates that adhesive wear might occur(Fig. 6(c)). Due to the localized high load at the small particles, thestress on some particles is postulated to be 8 GPa beyond the yieldstress. Thus some particles would deform and adhere to the sam-ples surface. Some particles indented deeper than at 300 N constantload, which could cause more severe abrasive wear. In Fig. 5(b),the OCP value oscillated during the tests, providing some evidence

Fig. 5. The OCP as a function of time for implants with 50 mg cobalt particles: (a) in0.3% NaCl at 300 N, (b) in 0.3% NaCl at 2 kN 300 N and (c) in 25% serum at 300 N and2 kN 300 N.

686 Y. Yan et al. / Wear 267 (2009) 683–688

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ig. 6. Optical images for implants tested in 0.3% NaCl: (a) without particles at 300s for 200 �m).

hat particles were crushed by the high load which resulted in evenmaller debris (5–20 nm). Particles may become more brittle as theyndergo a work hardening process by the mechanical force. There-

ore, instead of being spread to a flake shape, they were ‘crushed’nto small pieces.

Friction torque was measured during tests and the friction factoror the implants under two loading conditions and in two envi-onments is shown in Fig. 8. Some points can be summarized as

Fig. 7. TEM images for particles after tests in N

with particles at 300 N and (c) with particles at 2 kN 300 N (the magnification bar

following:

• With a high swing load, the friction factor is higher than under

300 N constant load.

• The friction factor in serum is much lower than in 0.3% NaCl solu-tion.

• At both loading conditions, added particles increased the frictionfactor.

aCl (a, b) at 300 N and (c) at 2 kN 300 N.

Y. Yan et al. / Wear 267 (2009) 683–688 687

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ig. 8. The friction factor for implants in two environments under two loading con-itions.

The ICP results for Co ions in the NaCl solution and in serumnder two loading conditions are shown in Fig. 9. Clearly by addingobalt particles, more metallic ions were released. The released ionsould be from three sources:

(a) the acceleration of the removal of the material from the bearingsurfaces due to the abrasive wear

b) the mechanical-induced release from the trapped particles and(c) pure dissolution/corrosion from the rest of the surface which

was not subjected to tribological contacts.

At higher load (2 kN 300 N), more cobalt ions were detected thannder 300 N. Moreover, it seems that in serum, at the same load-

ng condition, more metallic ions were released than in the salineolution, which is in an agreement with previous published resultsn a reciprocating tribometer [14–17]. Although the ion release pro-esses are chemical/electrochemical, because of their high inherentorrosion resistance, both bearing surfaces and cobalt particleselease very few metallic ions by pure dissolution. 50 mg cobalt par-icles were immersed in 0.3% NaCl and 25% serum for 20 min. 295±35) ppb cobalt ions were detected in saline solution and 1490±55) ppb cobalt in serum. It indicates that proteins can acceler-te the dissolution process of cobalt particles. It also shows thatechanical wear-induced dissolution/corrosion not only occurred

n the bearing surfaces but also on metallic debris.The effect of a different content of added particles on the OCP

nder 300 N is shown in Fig. 10. The OCP did not decrease as theoncentration of particles was increased. In contrast, a stabilizednd slightly more noble OCP was seen after 5 mg addition of cobaltarticles, which is the equivalent of debris generated after approx-

mate 600,000 cycles from a hip simulator [10]. This is consistentith the bedding-in theory. Studies have been conducted worldide on hip simulators and some clinical results, which show thattwo stages wear behaviour in terms of wear rate. A lower wear

ate regime called steady-state has been observed after 0.5–2 mil-ion cycles of a relatively higher wear rate bedding-in period [4–10].articles may act as rolling balls in the contact which resulted in less

amage on the bearing surface. It appeared that an equilibrium waseached according to the OCP results. However, in the hip simulatorr in clinical cases, generated debris does not remain in the bear-ng contact. It transfers from the head–cup contact to the capsulend can be disseminated around the body. Although cobalt parti-

ig. 9. ICP results for release cobalt ions in two solutions and under two loadingonditions.

Fig. 10. OCP as a function of time with different weight of particles at 300 N in (a)0.3% NaCl and (b) 25% serum.

cles were added in the clearance space between the head and thecup, it is very difficult to quantify the exact amount of particles inthe moving contact. Because of their small size (28 nm in diame-ter), gravity may play a minor role in the movement of the artificialparticles. More investigations will be carried out.

4. Conclusions

This paper investigates the abrasive wear mechanisms with adynamic implant simulator in biotribocorrosion systems. Addedcobalt particles acted as abrasives on the bearing surfaces. However,they were also affected by the mechanical motion and surroundingenvironments.

• Abrasive particles can cause local damage to the protective bar-rier (tribofilm), which resulted in an acceleration of metal ionsreleased from the bearing surfaces and increased friction.

• Under higher loading conditions, the wear mechanism trans-ferred from predominant abrasive-wear to the combination ofabrasive-wear and adhesive-wear.

• The dissolution of particles was enhanced by proteins in a staticcondition and under dynamic loading conditions.

• Wear-induced corrosion occurred on both bearing surfaces andparticles. Particles released more metallic ions under higher load.

• The system reached an equilibrium after more than 5 mg particleswere inserted. This may contribute to the bedding-in phase.

Acknowledgements

This study was financially funded by ARC, UK and the hip com-ponents were supplied by DePuy International Ltd.

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