Biotribological Behavior of Ultra High Molecular Weight

8/4/2019 Biotribological Behavior of Ultra High Molecular Weight

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Received 07 April 2008; accepted 12 June 2008

Projects 50535050 supported by the National Natural Science Foundation of China and NCET-06-0479 by the New Century Excellent Talents in University

Corresponding author. Tel: +86-516-83591915; E-mail address: [email protected]

Biotribological behavior of ultra high molecular weight

polyethylene composites containing bovine bone hydroxyapatite

LIU Jin-long, ZHU Yuan-yuan, WANG Qing-liang, GE Shi-rong

School of Materials and Engineering, China University of Mining & Technology, Xuzhou, Jiangsu 221008, China

Abstract: Wear particles of ultrahigh molecular weight polyethylene (UHMWPE) are the main cause of long-term failure of totaljoint replacements. Therefore, increasing its wear resistance or bioactivity will be very useful in order to obtain high quality artifi-cial joints. In our study, UHMWPE composites filled with the bovine bone hydroxyapatite (BHA) were prepared by the method ofcompression moulding. A ball-on-disc wear test was carried out with a Universal Micro-Tribometer to investigate the friction and

wear behavior of a Si3N4 ceramic ball, cross-sliding against the UHMWPE/BHA composites with human plasma lubrication. At the

same time, the profiles of the worn grooves on the UHMWPE/BHA surface were scanned. The experimental results indicate thatthe addition of BHA to UHMWPE had a significant effect on the biotribological behavior of UHMWPE cross-sliding against theSi3N4 ceramic ball. The addition of BHA powder enhanced the hardness and modulus of elasticity of these composites and de-creased the friction coefficients and wear rates under conditions of human plasma lubrication. When the added amount of BHA

powders was up to 20%~30%, UHMWPE/BHA composites demonstrated the designed performance of the mechanical propertiesand biotribological behavior.Key words: UHMWPE; hydroxyapatite; biotribology; composite material

1 Introduction

Ultra high molecular weight polyethylene

(UHMWPE) is an organic polymer widely used as anacetabular component in hip and total knee joint re-

placements, because of its excellent mechanical prop-erties, biocompatibility and stability in the humanbody

[12]. However, in recent years it has been recog-

nized that the wear debris of UHMWPE may be alimiting factor for the long-term success of prostheses.

At Present, most clinical results have revealed thataseptic loosening is the primary cause of failure of

total hip replacements (THRs) and accounts for al-most three-quarters of all revision operations

[36].

Aseptic loosening has been strongly linked with

UHMWPE wear debris-induced osteolysis and sincethe occurrence and severity of osteolysis appears tobe related to the size and concentration of wear parti-

cles, it follows that reducing the amount and rate ofgeneration of wear particles should reduce the occur-

rence of long-term aseptic loosening[78]

. Therefore, it

is believed that two effective methods to reduce asep-tic loosening are the following:

1) improving the wear resistance to reduce the

concentration of wear debris.

2) increasing the biocompatibility and bioactivityto decrease adverse biological reactions.

Since the wear resistance of UHMWPE appears to

be related to its mechanical properties, especiallythose on the surface, it follows that an increased en-

hancement of its mechanical properties should im-

prove the wear resistance of UHMWPE. Many au-thors have reported that the mechanical properties of

UHMWPE composites can be improved by introduc-ing reinforced second phases of ceramics (A12O3,

TiO2, quartz, wollastonite, etc.), metals or polymers

(polyurethane, phenylp-hydroxyzoate, etc.)[915]

. The

results show that these reinforced second phases mayimprove not only the mechanical properties but also

the wear performance of UHMWPE-based compos-ites. But, most of these filling materials have no or at

best poor biocompatibility and bioactivity.Hydroxyapatite (HA) is currently used as a bioma-

terial for many applications in biomedicine, becauseit can form a real bond with the surrounding bonetissue when implanted

[16]. Several studies on

UHMWPE/HA composites have been reported. Reis

et al. investigated the impact behavior of aHA/UHMWPE composite

[17]. Cunha et al. studied the

effect of processing conditions on the mechanical

J China Univ Mining & Technol 18 (2008) 06060612

JOURNAL OF CHINA UNIVERSITY OF

MINING &

TECHNOLOGY

www.elsevier.com/locate/jcumt


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LIU Jin-long et al Biotribological behavior of ultra high molecular weight polyethylene 607

behavior of the HA/UHMWPE composite using in-jection and compression molding

[18]. However, few of

these efforts are related to the natural bone hy-

droxyapatite, which has been used as bone implantmaterial due to its improved bioactivity, structure and

components, similar to that of human bones. Knets etal. produced an annealed compact bone tissue rein-

forced UHMWPE composite, but few processing de-

tails were given[19]

.In order to improve the wear resistance and bioac-

tivity of UHMWPE and reduce aseptic loosening,bovine bone hydroxyapatite (BHA) powders (


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Journal of China University of Mining & Technology Vol.18 No.4608

2.2 Friction and wear test

A ball-on-disc wear test was run on a Universal

Micro-Tribometer (Center Inc. USA) to investigatethe friction and wear behavior of the Si3N4 ceramic

ball, cross-sliding against UHMWPE/BHA compos-ites with human plasma lubrication (see Fig. 3a). The

reciprocating velocity of the upper sample is 8 mm/s,

reciprocating distance 0.2 mm and sliding speed of

lower sample 0.21 m/s. In sliding bearing, sliding

wear in a rotational direction is normally noticed. But

the bending deflection in the axial direction willcause friction and wear. So, the friction and wear be-

havior of UNMWPE/BHA composites under cross-sliding direction conditions have been studied for the

movement of artificial knee joints and hip joints, in-

cluding tangential and radial loads.

(a) Schematic diagram (b) Wear track

Fig. 3 Schematic diagram of abrasion testing and wear track on UHMWPE/BHA composite disc

The Si3N4 ceramic ball was 4 mm in diameter and

fixed on the load arm. The UHMWPE/BHA compos-ite disc was rotated by an electrical motor controlled

by a frequency converter. Fig. 3b shows that, when

the disc is rotated along the circumference of a circle,the Si3N4 ceramic ball cross-slid ten times. The con-

tact load was 3.9 N and the sliding speed 0.21 m/s.The testing time was 120 min in each test. All ex-

periments were performed in human plasma lubrica-

tion at room temperature. The relative humidity was50%~60%. Each wear test was repeated at least three

times. Prior to the wear testing, each of the

UHMWPE composite discs was ground to obtain asmooth surface. Grinding was performed in steps

with abrasive papers of decreasing grit size. Theground surfaces were polished with a diamond paste

with particle sizes of 0.35 m on a nylon polishing

cloth. The average surface roughness, Ra of the pol-ished surfaces was less than 0.2 m, measured using

a conventional profilometer.In order to normalize the wear results in our study,

the commonly used wear rate k was calculated bydividing the reduced wear weight m (BP211D elec-tron analytical balance with 0.01 mg precision,

Sartorius AG, Germany) by the product of total

sliding distance S, the applied normal load Nandthe density of the material

[20]: k =m /NS.

Analysis of variance using regression analysis andStudents t-test was carried out to test for significant

differences between various sets of experimental data.A probability level ofp


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Fig. 4 Effect of amount of BHA on hardness and modulusof elasticity of UHMWPE/BHA composites

3.2 Friction coefficients

The friction coefficients of UHMWPE/BHA com-

posites of Si3N4 with human plasma lubrication at

room temperature are shown in Fig. 5a. Each of theUHMWPE/BHA composites had the same change in

the friction coefficients over time. The friction coeffi-

cients decreased rapidly for a short time of slidingand then tended to reach a steady state at about 60

min. The variation of the friction coefficient for all

materials fluctuated little during the entire wearprocess. From the average friction coefficients in Fig.

5b, it is seen that the addition of BHA clearly de-creased the friction coefficients of the UHMWPE/

BHA composites against Si3N4. When the amount of

BHA was up to 20%~30%, the lowest friction coeffi-cient was obtained. For example, the friction coeffi-

cient of UHMWPE was approximately 0.064, butonly about 0.042 and 0.048 for UHMWPE/20% BHA

and UHMWPE/30% BHA composites respectively,

which is only about 60% of that of UHMWPE.However, when the increased amount of BHA ex-ceeded 30%, the friction coefficient values increased

with the increment in the amount of BHA.

(a) Friction coefficients (b) Steady friction coefficients

Fig. 5 Friction coefficients of UHMWPE/BHA cross-sliding against Si3N4 ceramic in human plasma lubrication and steady

friction coefficients acquired by calculating the average friction coefficients between 60 and 120 min of sliding time

3.3 Wear behavior

The bar charts in Fig. 6 present the wear rates ofUHMWPE/BHA composites with human plasma lu-

brication. It clearly shows that the addition of BHA to

UHMWPE can raise the wear rates of these compos-

ites. When the amount of BHA was less than 30%,the wear rate of the composite would reduce with anincrease in the amount of BHA. The sample of

UHMWPE/20% BHA material had a wear rate of7.6110

8mm

3/Nm, which was only about 60% of

the 12.38108

mm3/Nm of UHMWPE. It also shows

that the wear rate increases with an increase in the

amount of BHA when it exceeds 30%. But, these de-creases were clearly less than that of UHMWPE. In

addition, under the cross-sliding movement the wearrate of UHMWPE was higher than that in a single

bearing slide[21]

.

Worn grooves on UHMWPE/BHA composite sur-faces are presented in Fig. 7. Although it was difficult

to position the boundary of the worn groove exactly,

these worn-out areas still demonstrate that worn

grooves from UHMWPE were deeper or wider thanthose of UHMWPE/BHA composites. Composites

with lower wear rates show narrower and shallower

worn grooves, which agreed with the wear rates inour tests.

Fig. 6 Comparison on wear rate of UHMWPE/BHA

composites cross-sliding against Si3N4


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m

m

m

m

m

m

m

m

m

m

m

m

Fig. 7 Profiles of worn grooves on UHMWPE/BHA composite surfaces with human plasma lubrication

4 Discussion

The experimental results from our tests indicatethat the addition of BHA to UHMWPE had a signifi-

cant effect on the wear and friction behavior of

UHMWPE cross-sliding against the Si3N4 ceramicball. It clearly showed that the addition of BHA pow-

der enhances the hardness and modulus of elasticityof these composites and decreased both the friction

coefficients and wear rates with human plasma lubri-cation. When the amount of BHA reached 20%~30%,the UHMWPE/BHA composites demonstrated thedesigned performance of the mechanical propertiesand biotribological behavior.

It is well known that the friction force of

UHMWPE polymers against ceramic balls is com- posed of interfacial adhesion and ploughing forces.

The adhesion force is affected by the effective

modulus of elasticity, i.e., there is an inversely proportional relationship. The ploughing force depends

on the shear strength of UHMWPE surfaces. It is seenfrom Fig. 4 that the modulus of elastiocity of the sur-face of UHMWPE/BHA composites increases along

with enhanced surface hardness. The modulus ofelasticity of the surface increased from 720 to 1077

MPa, while the hardness increased from 117 to 133MPa. In some experiments

[22]it was reported that the

gamma irradiated UHMWPE has a modulus of sur-

face elasticity of approximately 1200 for untreatedspecimens, 1450 for specimens treated with 75 kGy

and 1530 MPa for a treatment of 150 kGy, corre-

sponding to a surface hardness of about 37, 46 and 54,respectively. (The variation in the data for UHMWPEmight be related to the different method of prepara-

tion). For this reason, the increment in the effectivemodulus of elasticity might reduce the adhesive force

of the Si3N4 ceramic ball on UHMWPE/BHA com-

posite surfaces. At the same time, the hardened sur-face can increase the ploughing force against the hard

asperity of the surface of the Si3N4 ball with the

hardness enhancement of the UHMWPE/BHA composite surface. Consequently, the summation of the

adhesion and ploughing forces resulted in the lowfriction coefficients for UHMWPE/BHA composites

in Fig. 5.

The pictures in Fig. 8 show the two worn surfaces

of UHMWPE and UHMWPE/20% BHA. It is seenfrom Fig. 8a that the worn surface of UHMWPE hasa rough worn appearance, including adhered debris,

serious grooves and fatigue fractures, almost all on

the worn surfaces. For UHMWPE/BHA composite,the worn scars have become less so, owing to its sur-face hardness enhancement. The smoother worn sur-

face appears by reason of the shallow grooves. Theadhered debris and fatigue fractures could be found

only in one small worn-out area in Fig. 8b. Theseresults are likely caused by the decrease in the wear

rate of this material. When the Si3N4 ball was rubbing

on the UHMWPE/BHA surface, the wear resistance

of polymer was inversely proportional to its hardness.With an increase in hardness and the modulus of elas-ticity shown in Fig. 4, the enhancement of ploughingand adhesive wear resistance could have resulted in

the lower wear rates of UHMWPE/BHA composites.Meanwhile, low wear rates of UHMWPE against

the ceramic ball were contributed by the deposition of

protein and other organic contents in plasma from the plasma lubrication. Some authors reported that the

serum protein concentration had a significant effecton the wear performance of CoCr alloys to PTFE

polymer and the film deposited by the serum protein

might affect the wear of UHMWPE[2325]. For theUHMWPE/BHA composites, the biocompatibility

and bioactivity of BHA resulted in a good bonding

ability to the mass of the proteins in human plasma,


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which greatly increased the ability of these compos-ites to absorb the plasma protein. The concentration

of the protein deposition enhanced with an increase of

the amount of HA[23]

. The protein deposition in hu-man plasma lubrication made it possible to form a

thin protective film, reducing the abrasion of theSi3N4 ball on UHMWPE surface. Furthermore, lower

wear rates of UHMWPE/BHA composites were ob-

tained in human plasma lubrication and the greaterthe additive amount, the lower the wear rates. How-

ever, when the amount of BHA was greater than the

optimum amount, the wear resistance of UHMWPE/BHA composite decreased due to their lower hard-

ness and modulus of elasticity, as shown in Figs. 4and 6.

(a) UHMWPE (b) UHMWPE/BHA 20%

Fig. 8 SEM micrographs of worn surfaces of UHMWPE cross-sliding against Si3N4 ceramic

5 Conclusions

1) The addition of BHA powder to UHMWPE hasshown to be beneficial for increasing its mi-

cro-surface mechanical properties. When the amount

of BHA was less than 30%, the hardness and modulusof elasticity clearly improved. For the UHMWPE/

20%BHA composite, the micro-surface hardness and

modulus of elasticity were about 15% and 50%higher than that of pure UHMWPE.

2) With human plasma lubrication, the addition ofBHA to UHMWPE was effective in decreasing the

friction coefficients and wear rates of UHMWPE/

BHA composites cross-sliding against the Si3N4 ce-ramic ball. Reduction of 40% in the wear rate was

obtained for the UHMWPE/20% BHA composite.3) Ploughing and fatigue fracture were the main

wear mechanisms of the UHMWPE/BHA composites

cross-sliding against the Si3N4 ceramic with humanplasma lubrication.

Acknowledgements

The authors wish to thank for the support from Na-

tional Nature Science Foundation of China(50535050) and Program for New Century Excellent

Talents in University (NCET-06-0479).

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Biotribological Behavior of Ultra High Molecular Weight