Feasibility Study of Novel Low Temperature Degradation-free … · 2013-08-04 · Feasibility Study...

Biomaterials Research (2011) 15(3) : 105-113

105

Biomaterials

Research

C The Korean Society for Biomaterials

Feasibility Study of Novel Low Temperature Degradation-free Zirconia/Alumina Composites for Total HIP Replacements

Dae-Joon Kim1*, Kwon-Yong Lee2, Hee-Joong Kim3, and Jung-Suk Han4

1Department of Advanced Materials Engineering, Sejong University, 98 Kunja-dong, Seoul 143-747, Korea2Department of Mechanical Engineering, Sejong University, 98 Kunja-dong, Seoul 143-747, Korea3Department of Orthopedic Surgery, School of Medicine, Seoul National University, Seoul 110-744, Korea4Department of Prosthodontics, School of Dentistry, Seoul National University, Seoul 110-749, Korea(Received May 20, 2011/Acccepted August 1, 2011)

Feasibility of new zirconia/alumina composites, based tetragonal zirconias containing 5.3 mol% Y2O3 and 4.5 mol%Nb2O5 and 3.0 mol% Y2O3, 1.6 mol% Nb2O5, and 3.6 mol% CeO2, for total hip replacements was evaluated in termsof mechanical properties, tetragonal phase stability under autoclaving conditions, wear behaviors, biocompatibility, andsterilization characteristics. Optimal 4-point flexural strength and fracture toughness of the composite were 930 MPaand 12.5 MPam1/2, respectively, and the tetragonal phase in the composites did not transform to the monocliniczirconia after autoclaving for 10 h at 200oC. Bovine serum lubricated sliding wear tests of the composite against thecomposite and the composite against UHMWPE indicated excellent wear resistance especially in alumina-richcomposites. Either bulk or particle forms of the composites showed at least equivalent or slightly better biologicalresponse than pure titanium. 50-kGy gamma irradiation was an optimal sterilization method for the zirconia/alu-mina composites. These results suggest that these novel zirconia/alumina composites can be alternative bearingoptions for total hip replacements.

Key words: Zirconia, degradation, wear, biocompatibility, sterilization

Introduction

otal hip replacements eliminate pain due to debilitating

diseases such as osteo- and rheumatoid arthritis, avascular

necrosis, bone cancer, and trauma, and dramatically return

mobility and functionality to the joint. The longevity of artificial

joints is influenced mainly by the wear rate of UHMWPE

acetabular cups articulating against joint heads. For the sake of

minimizing the wear debris, ceramics have been employed as

the head materials. Zirconia heads are used in about 20% of

the total number of operations per year in Western Europe,

which is around 360,000,1) and 6% of the hip implant pro-

cedures in the United States, which roughly equals 150,000 to

200,000.2) It has been reported that more than 600,000 zirco-

nia hip joint heads have been implanted worldwide since

1985.3)

Most of the zirconia for the hip joint heads consists of

97 mol% ZrO2 and 3 mol% Y2O3 (3Y-TZP). Recently, however,

due to the low temperature degradation (LTD) of the zirconia,

there have been questions about the long term stability and

wear performance of the zirconia bioceramic. A retrieval study

of the yttria-stabilized tetragonal zirconia polycrystal balls

showed that 20-30% of tetragonal phase was transformed to a

monoclinic phase after 3-6 years implantation.4) This transfor-

mation is followed by surface roughening, grain pull out and

micro-cracking,5) resulting in increasing wear and possible pre-

mature failure. LTD is known to occur as a result of the spon-

taneous phase transformation of the tetragonal zirconia to the

monoclinic phase during aging at 100-300oC. The interaction

of microcracks accompanied by the phase transformation leads

to the formation of macrocracks, which results in an abrupt

decrease in strength. Since LTD is governed by the kinetics of

the phase transformation6) and accelerated in the presence of

water and water vapor,7) it is likely to occur even in the human

environment when the zirconia heads have been implanted for

a long period of time. Besides LTD, which is currently the

primary concern, some case studies of zirconia indicated that

delayed failure could also occur in vivo by a slow crack growth

(SCG) process.8) SCG in bioceramics is attributed to the com-

bined effect of high stress at the crack tip and the presence of

water or body fluid molecules that induce crack propagation in

a subcritical manner (below the toughness). Thus, there is a

need today to develop degradation-free and crack-resistant zir-

conia ceramics to improve the lifetime and the reliability of

T

*Corresponding author: [email protected]

106 Dae-Joon Kim, Kwon-Yong Lee, Hee-Joong Kim, and Jung-Suk Han

Biomaterials Research 2011

prostheses. In this study, feasibility of novel LTD-free zirconia/

alumina composites for total hip replacements was evaluated in

terms of biocompatibility, low temperature phase stability, mech-

anical and wear properties, and sterilization capability of gamma

irradiation and ethylene oxide gas.

Phase Stability and Mechanical Properties of LTD-free Zirconia Composites

90.2 mol% ZrO2-5.3 mol% Y

2O

3-4.5 mol% Nb

2O

5 Tet-

ragonal Zirconia Solid SolutionIn Table 1, the tetragonal phase stability under the autoclave

treatment at 250oC for 5 h and the mechanical properties of

the degradation-free tetragonal zirconia, the composition of

which is 90.2 mol% ZrO2-5.3 mol% Y2O3-4.5 mol% Nb2O5

((Y,Nb)-TZP), were compared with those of 3 mol% Y2O3 (3Y-

TZP), which is most widely used for the zirconia hip joint

heads. Both specimens were obtained by ambient sintering for

2 h at 1550oC. Due to the intrinsic phase stability of (Y,Nb)-TZP,

the specimen showed no degradation. On the other hand,

most of the tetragonal phase in 3Y-TZP was transformed to the

monoclinic phase, indicating the material is vulnerable to LTD.

Although several mechanisms responsible for LTD have been

proposed, there is a general agreement that LTD is a relaxation

process of internally strained tetragonal zirconia (t-ZrO2) lattice

by a thermally activated oxygen ion diffusion.6,9,10) The internal

strain stems from the fact that the ionic size of Zr4+ is too small

to be coordinated to eight oxygen ions in the fluorite structure.

The metastability of the 3Y-TZP is ensured by the substitution of

trivalent Y3+, whose ionic size is larger than that of Zr4+, for

tetravalent Zr4+. Besides the compensation for the size mis-

match, the oxygen vacancies, formed as a result of the substitu-

tion, also contribute to the metastability because they allow for

a fraction of Zr4+ ions to take ZrO7 oxygen polyhedron. Ther-

mal expansion anisotropy, which governs the grain size depen-

dence of transformability of t-ZrO2, is another source of internal

strain. The degree of strain in the tetragonal lattice is propor-

tional to the c/a axial ratio of the tetragonal structure, which

manifests the transformability.10) As schematically presented in

Figure 1, the oxygen vacancy diffusion along a stress gradient

from highly stressed specimen surface of 3Y-TZP to less stressed

interior, results in the cation network being strained and the

oxygen ions being overcrowded at the surface. This action leads

to a highly strained t-ZrO2 lattice and, consequently, phase

instability of t-ZrO2 solid solutions. The tetragonal to monoclinic

phase transformation that induces LTD proceeds to relieve the

residual stress, as the stress becomes sufficient to overcome the

nucleation barrier to monoclinic zirconia formation with

prolonged aging.

The (Y,Nb)-TZP has a relatively stable lattice since the Nb5+

ion, having a smaller ionic size than Zr4+, has 4 coordination

with the oxygen ions in t-ZrO2, which leads to a localized Y-

Nb ordering for a scheelite structure.11) This ionic arrangement

relieves the internal strain. Furthermore, the substitution of

Nb5+ for Zr4+ annihilates the oxygen vacancies formed by the

Y3+ substitution, which makes the vacancy diffusion less avail-

able. The stable lattice structure and the low vacancy concen-

tration in the (Y,Nb)-TZP led to the degradation-free tetragonal

zirconia as shown in Table 1. On the other hand, 3Y-TZP pos-

sesses a strained lattice, which promotes the diffusion, and a

considerable number of the oxygen vacancies. These factors

caused the phase transformation of 87% of the tetragonal

phase in Table 1. Since 3Y-TZP consists of ~87% tetragonal

and ~13% cubic phases, the monoclinic phase of 87% means

the complete transformation of the tetragonal phase in 3Y-TZP.

The addition of rigid alumina of the modulus of about 400

GPa into 3Y-TZP of the modulus of about 220 GPa suppresses

the relaxation of the tetragonal lattice so that the extent of the

degradation may decrease under a mild aging environment.

However, the effect was not significant under severe aging

conditions such as the autoclave treatments in this study,

which only a slight decrease to 73% was observed.

The strength of (Y,Nb)-TZP was lower than that of 3Y-TZP in

Table 1 since the strength is inversely proportional to the grain

size as shown in Figure 2. However, the addition of 20 vol% of

Al2O3, the particle size of which is 2.8 µm, drastically increased

both the biaxial strength and the toughness of the (Y,Nb)-TZP

from 528 and 5.9 to 690 MPa and 8.2 MPam1/2, respectively.12)

The improvement in strength is mainly due to the increase in

the toughness according to the linear elastic fracture mechanics

and partially due to the grain size refinement of the (Y,Nb)-TZP

since the alumina particles behave as an inhibitor of the zirco-

nia grains during sintering. The increase in the toughness arises

from the contributions of the phase transformation toughening,

enhanced by the residual stress as a result of the thermal

expansion mismatch between the zirconia and the alumina

particles, and the crack bridging toughening by the alumina

particles. Elevation of the sintering temperature of the compos-Figure 1. Schematics of proposed mechanism for low temper-ature degradation.

Table 1. Monoclinic ZrO2 content after annealing for 5 h at 250oC

and 3.97 MPa water vapor pressure, biaxial strength, and fracture toughness of 3Y-TZP and (Y,Nb)-TZP

m-ZrO2[%] Strength, MPa Toughness, MPam1/2

3Y-TZP 87 850 6.1

(Y,Nb)-TZP 0 528 5.9

Feasibility Study of Novel Low Temperature Degradation-free Zirconia/Alumina Composites for Total HIP Replacement 107

Vol. 15, No. 3

ite to 1650oC slightly decreased 4-point flexural strength to 620

MPa, but probably due to enlargement of the microstructure,

significantly increased the toughness to 10.5 MPam1/2. The 4-

point flexural strength and the toughness were further improved

to 930 MPa and 12.5 MPam1/2 by employing hot isostatic

pressing (HIP) at 1450oC for 30 min. The HIP processing dra-

matically improved the reliability of the composite as evidenced

by the increase in the Weibull modulus from 9 to 33 as shown

in Figure 3.

The (Y,Nb)-TZP/alumina composite was machined to the hip

joint heads with the sphericity of 1.0 µm and the surface rough-

ness (Ra) of 0.02 µm. The quality of machining surpasses the

FDA requirements for the sphericity and the roughness of

ceramic hip joint balls, which are < 5 µm and < 0.2 µm, re-

spectively. The excellent phase stability, mechanical properties,

biocompatibility, and machinability allow the composite to be

a candidate material for the ceramic hip joint heads

91.8 mol% ZrO2-3 mol% Y

2O

3-1.6 mol% Nb

2O

5-3.6 mol%

CeO2 tetragonal zirconia solid solution

The addition of Nb2O5 to 3Y-TZP does not change its flexural

strength, but significantly increases toughness as a result of

enhanced transformability of tetragonal to monoclinic phase.6,13)

However, because the high transformability corresponds to the

highly strained tetragonal lattice,6) any attempt to improve

toughness causes more extensive LTD. In an effort to suppress

LTD, Al2O3, having 0.3-0.5 µm particle size, was added into the

t-ZrO2 solid solution, which consists of 95.4 mol% ZrO2-3 mol%

Y2O3-1.6 mol% Nb2O5 ((3Y,1.6 Nb)-TZP), to form zirconia/alu-

mina composites.14) As show in Figure 4, the biaxial strength of

the specimens was higher than 1 GPa prior to the aging for 10

Figure 2. SEM micrographs of (a) 3Y-TZP, (b) (Y,Nb)-TZP, and (c) (Y,Nb)-TZP/10 vol% Al2O

3 composite.

Figure 3. Influence of hot isostatic pressing on Weibull plots of80Zr/20Al composite.

Figure 4. Flexural strength of alumina/zirconia composites beforeand after aging for 10 h at 200oC in autoclave.

Figure 5. Monoclinic phase content in alumina/zirconia com-posites after aging for 10 h at 200oC in autoclave.



h 200oC. After the treatment, however, the strength dropped

abruptly, especially for the specimens having the zirconia more

than 45 vol%, as a result of the extensive phase transformation

as shown in Figure 5. The surfaces of the extensively trans-

formed specimens were covered with cracks after the anneal-

ing. On the other hand, the strength of the specimens having

the zirconia less than 40 vol% was not influenced by the treat-

ment because the integrity of the alumina matrix was main-

tained even after the transformation. The aging treatment of

200oC and 1.55 MPa for 10 h is very severe, since, from time-

temperature equivalence,15) it would represent roughly one

thousand years at 37°C. However, it should be kept in mind

that hip component wear can significantly increase surface

temperature, and that any degradation in vivo has to be

avoided. This treatment is, therefore, a conservative approach.

To find a degradation-free zirconia having high mechanical

properties, this study included an investigation of the influence

of CeO2 on the extent of LTD of (3Y,1.6 Nb)-TZP. Although the

addition of Nb2O5 aggravates LTD of 3Y-TZP due to the

increase in the c/a axial ratio of tetragonal lattice, the addition

of CeO2 is likely to alleviate the degradation since CeO2 de-

creases the axial ratio.16) The extent of degradation of 3Y-TZP

containing Nb2O5 was slightly decreased as the CeO2 content

increased up to 3.5 mol% and then dropped abruptly as the

content reached to 4 mol% as shown in Figure 6, where each

CeO2 composition in the legend comprises six specimens in

which Nb2O5 content ranges from 0.6 to 1.6 mol% in the inter-

val of 0.2 mol%. The relationship between the extent of degra-

dation and the fracture toughness of 3Y-TZP containing Nb2O5

and CeO2 in Figure 6 indicates that the specimens with high

fracture toughness are apt to transform to monoclinic phase

while aging. The minimum amount of CeO2 for stabilization of

(3Y,1.6 Nb)-TZP during the aging procedure is 3.6 mol% as

shown in Figure 6. A further increase in the CeO2 content,

which increases in both grain size and tetragonal phase stability,

causes a decrease in strength and toughness. Thus, the 3Y-TZP

containing 1.6 mol% Nb2O5 and 3.6 mol% CeO2 ((Y,Nb,Ce)-

TZP) gave the highest mechanical properties while suppressing

the degradation. The addition of Nb2O5 and CeO2 did not

affect biocompatibility of 3Y-TZP.

(Y,Nb,Ce)-TZP was mixed with Al2O3, varying the contents

from 10 to 90 vol% to form (Y,Nb,Ce)-TZP/Al2O3 composites.

Biaxial strength of the composites was increased from 740 MPa

to more than 880 MPa by the addition of 20-30 vol% Al2O3

and then decreased by further additions as shown in Figure 7.

Due to its high Young’s modulus and low thermal expansion

coefficient when compared with zirconia, alumina enhances

strength and toughness by a high stress-induced phase transfor-

mation of zirconia. In Figure 7 these contributions reach a max-

imum at about 20-30 vol% of Al2O3 and then, since the

fraction of tetragonal zirconia in the composites plays a major

role in determining the mechanical properties in this composi-

tion range, become insignificant with a further increase.

Slow crack growth behaviors of LTD-free zirconia/alu-mina composites

For the determination of SCG behavior, (Y,Nb,Ce)-TZP/20

vol% Al2O3 (C1) was selected to match the Al2O3 content in

(Y,Nb)-TZP/20 vol% Al2O3 (C2) that has been successfully used

for ceramic abutments of dental implants and ceramic femoral

heads in total hip replacements.17) SCG behaviors of C1 and C2

were determined from the crack growth rate (V) vs. stress inten-

Figure 7. Variation of biaxial flexural strength of (Y,Nb,Ce)-TZP/Al

2O

3 composite as a function of Al

2O

3 content.

Figure 6. Relationship between toughness and extent of (Y,Nb,Ce)-TZP.


Vol. 15, No. 3

sity factor (KI) diagrams.18) These results were compared with

conventional 3Y-TZP and alumina ceramics used today in ortho-

pedics. It was clear that C1 exhibits the best SCG resistance, the

V-KI diagram being shifted to higher stress intensity factors as

compared with C2 and the monoliths. For the same stress

intensity factor value, the crack growth rate was about one

order, two orders, and 4 orders of magnitude lower than with

C2, conventional 3Y-TZP, and alumina, respectively. Thus, C1

exhibits both increased fast crack growth resistance (higher

toughness) and SCG resistance. Measurements are undergoing

to investigate the threshold values below which no crack prop-

agation occurs.

In addition to the suppression of LTD and high mechanical

properties (strength and toughness), a significant improvement

of SCG resistance of C1 was observed, since its SCG is only

operant at higher KI. C1 exhibits roughly the same SCG resis-

tance as that processed by De Aza et al.,19) with a composition

in the high alumina content domain (90% alumina and 10%

3Y-TZP). It is therefore possible to tailor the composition and

the microstructure of (Y,Nb,Ce)-TZP/alumina composites, in the

high zirconia content domain, exhibiting both LTD and SCG

resistance together with a strength of more than 800 MPa,

without using a hot isostatic pressing. Compared to C2 and alu-

mina and zirconia monoliths, this composite should be the

optimum choice for longer lasting, reliable implants. Further

SCG thresholds, wear tests, and in-vivo studies should be con-

ducted to confirm the feasibility of such composites, but they

already appear as good candidates as far as dental or orthope-

dic applications are concerned.

Wear Characteristics of LTD-free zirconia/alu-mina composites

Sliding wear behaviors of UHMWPE against ceramicsPin-on-disc sliding wear tests (n = 3) were conducted with

the polyethylene pins against ceramic disks of alumina, zirconia,

and composite containing 80 vol% of (Y,Nb)-TZP and 20 vol%

Al2O3 (80Zr/20Al) in bovine serum at room temperature. Disks

were moved in the two different kinematic motions of linear

reciprocal sliding and repeat pass rotational sliding. A lever

arrangement and a dead weight of 315 N exerted a nominal

contact pressure of 4 MPa, which is equivalent to average con-

tact pressure in the hip joint for the normal gait. A frequency of

1 Hz produces a sliding velocity of 62.5 mm/s at the center of

the cylinder specimen for both kinematic motions. All tests

were interrupted after every ten thousand cycles, the specimen

was cleaned with deionized water, dried with a tissue, and

weighed with a microbalance (sensitivity of 0.01 mg). Wear

testing was continued for one million cycles, or the equivalent

to a total sliding distance of 62.5 km. The amount of wear was

determined by weight loss of each pin specimen, which was

corrected for the weight gain that was obtained from soak con-

trol tests.

After one million cycles of sliding, mean weight losses (wear)

of UHMWPE pins against alumina, zirconia, and 80Zr/20Al

disks were plotted and were shown in Figure 8. In the view-

point of two different kinematic motions, the wear of UHM-

WPE pins tested in a linear reciprocal motion (LRM) was 2~3

times higher than that in a repeat pass rotational motion

(RPRM) for all ceramic disks. This means that the repeated large

directional change of contact stresses generates more wear in

polyethylene, and, therefore, the wear of the polyethylene is

very sensitive to the relative motion between two contact sur-

faces. The wear of UHMWPE pins against the novel 80Zr/20Al

composite disks was about 50% lower than that of conventional

zirconia and alumina disks in a linear reciprocal motion and a

repeat pass rotational motion. This trend is much lower than SS

316L disks which were employed as a control.

Contact wetting angles of disks of alumina, zirconia, and

80Zr/20Al composite were 75o, 67o, and 67o, respectively (Fig-

ure 9). 80Zr/20Al composite has an equivalent wetting angle to

zirconia and lower than alumina, indicating that the composite

has a good lubrication character. This partially rationalizes the

RPRM wear test results in Figure 8.

Sliding wear behaviors of ceramic to ceramic contactpairs

Disk and cylinder-type specimens for sliding wear tests were

prepared from the composite containing 20 vol% of (Y,Nb)-

Figure 8. Wear of UHMWPE pins against all ceramic disks aftersliding tests for one million cycles.

Figure 9. Contact wetting angles of alumina, zirconia, and 80Zr/20Al composite.



TZP and 80 vol% Al2O3 (20Zr/80Al) and 80Zr/20Al composite.

The wear tests were performed using a pin-on-disk type wear

tester in a linear reciprocal sliding motion with a sliding distance

of 10 mm per cycle at a frequency of 1 Hz. All specimens were

tested with a line contact between lateral surface of cylindrical

pin and flat surface of disk in both dry and bovine serum lubri-

cated conditions at room temperature. In dry sliding wear tests,

a load of 75 N was exerted to contact areas on the specimen,

and in the bovine serum lubricated sliding wear tests, two levels

of loads of 150 N and 225 N were exerted.

Figure 10 shows that the grain size of 20Zr/80Al is much

finer than 80Zr/20Al. After dry sliding of 4 × 104 cycles, no

severe wear damage was observed in 20Zr/80Al, except some

fine debris and pitting defect, which might have occurred from

the specimen preparation procedure (Figure 11(a)). On the

other hand, as shown in Figure 11(b), large scale cracks formed

in 80Zr/20Al. Near the fractured region the surface layer was

sunk down locally and then this layer was smashed into pieces

under repeated applied load. As a result of this consecutive

action during long term sliding contacts, many smashed layers

were chopped into a large amount of debris in the form of

multi-layered cloud. In the results of the dry sliding wear tests,

20Zr/80Al, having a fine grain size, strength of 700 MPa, and

toughness of 6.3 MPam1/2, exhibited higher wear resistance in

a ceramic-ceramic contact pair than 80Zr/20Al composite, the

strength and toughness of which are 860 MPa and 8.5 MPam1/2

respectively. It is likely that strength and fracture toughness are

not influential to the wear resistance, but that hardness and

microstructure are.

For the entire duration of the bovine serum lubricated sliding

wear tests, almost no wear track or debris was observed, using

optical and confocal microscope, on all disk specimens as de-

termined by optical and confocal microscope. In this case, the

applied loads were up to three times larger than the load

exerted in the dry sliding tests. Wear was too little to be

measured but some wear debris were observed from the bovine

serum by optical microscope. Nevertheless, grain loosening

followed by grain boundary cracking, pitting, and large scaled

cracks were detected, from the morphological observation by

SEM, on the wear tracks of both composites. This indicates that

debris formation is inevitable, even if the composites have

excellent wear resistance.

Wear characteristics of ceramic to ceramic in a pointcontact

LTD-free 80Zr/20Al and 20Zr/80Al composites were die-

pressed into a disk and a half-sphere shape cylinder, and then

isostatically pressed at 140 MPa. The compacts were sintered

for 2 h at 1550oC in air. Sintered disks (10 thick 57 mm diam-

eter) and cylinders (9 mm long 10 mm diameter) were ground

and polished to a surface roughness of Ra < 0.03 µm. The wear

tests were performed using a pin-on-disk type wear tester in a

linear reciprocal sliding motion, with a sliding distance of 10

mm per cycle, at a frequency of 1 Hz. All specimens were

tested with a point contact between the summit of cylindrical

pin and flat surface of disk in both dry and bovine serum lubri-

cated conditions at room temperature. In dry and bovine serum

lubricated sliding wear tests, loads of 30 N and 60 N were

exerted, respectively, to the contact area on the specimen.

No appreciable weight loss was observed from both pin

specimens after dry sliding tests for 15,000 cycles. However, the

damages (visible in Figure 12) formed on wear tracks and the

depth profiles of the tracks (Figure 13) indicate that the com-

posite containing large amount of Al2O3, which has higher elas-

tic modulus than zirconia, possesses more enhanced wear

resistance because of its fine microstructure and a high hard-

ness. On the other hand, bovine serum lubricated sliding wear

tests for 30,000 cycles showed no damage on both wear tracks

of the disks and the pins. Nevertheless, as shown in Figure 14,

ring-shaped cracks were observed on the 80Zr/20Al disk after

Figure 10. SEM micrographs of LTD-free zirconia/alumina com-posites.

Figure 11. SEM images of surface damages on wear tracks ofceramic-on-ceramic line contact after dry sliding wear tests for4 × 104 cycles at a load of 75 N.

Figure 12. Photographs showing wear tracks on the disk specimensof ceramic-on-ceramic point contact.


Vol. 15, No. 3

the test, indicating that the 20Zr/80Al is a more reliable mate-

rial for the orthopedic applications.

Biocompatibility of LTD-free zirconia composites

Initial bone cell responseBiocompatibility of 80Zr/20Al composite has been evaluated

based on the toxicity17) and the initial osteoblast-like cell re-

sponse.20) The initial bone cell response to 80Zr/20Al compos-

ite has been examined from cell morphology, cell proliferation

analysis (MTS), alkaline phosphatase (ALP) activity, and m-RNA

of integrin β1 activity. SEM observations, as shown in Figure 15,

reveal that the osteoblast-like cells are well spread and attach

onto the composite. The results of the MTS assay (Figure 16)

indicate that cell proliferation increases with increasing culture

time and the cells on the composite are more active in prolifer-

ation compared to titanium, which is known as a biocompatible

material. Osteoblast differentiation generally implies the increase

in ALP activity and specific proteins expression such as osteocal-

cin, osteopontin, and type I collagen. In this experiment, as

shown in Figure 17, the ALP activity increases with increasing

the culture time. Integrin β1 is known as the major integrin sub-

unit involved in osteoblast adhesion on biomaterials and, by

mediating several attachment proteins, in initial cell attachment.

As indicated in Figure 18, the activity of m-RNA of integrin β1 is

Figure 14. SEM images of surface damages on wear tracks ofceramic-on-ceramic point contact after lubricated sliding testsfor 30,000 cycles.

Figure 13. Depth profiles of wear tracks on the disk specimensof ceramic-on-ceramic point contact measured by a surface pro-filometer after 15,000 cycles.

Figure 15. SEM observations of HOS cells on 80Zr/20Al composite as a function of culture time.

Figure 16. Cell proliferation analysis (MTS) of HOS cells seededfor 1, 4, and 8 days onto titanium (Ti) and 80Zr/20Al composite(Zc).

Figure 17. ALP activity of HOS cells seeded for 1, 4, and 8 daysonto titanium (Ti) and 80Zr/20Al composite (Zc).

Figure 18. RT-PCR amplification of mRNAs in total RNA lysatesfrom HOS cells to assess integrin β1 expression at different earlytime points of cell proliferation : (A) 80Zr/20Al 12h; (B) 80Zr/20Al 24h; (C) 80Zr/20Al 48h; (D) Ti 12h; (E) Ti 24h; (F) Ti 48h.



higher on the composite than on titanium specimens. This sug-

gests that the composite enhances cellular adhesion with minor

changes in the gene expression of the osteoblast-like cells.

In vivo response to the wear debris As indicated in Figure 19, although the bone tissues exposed

to the wear debris of 20Zr/80Al, 80Zr/20Al, and 80(Y,Nb,Ce)-

TZP/20Al composites shows moderate inflammatory reactions,

fibrosis, and granulations with osteolysis at 1 week, these tissue

reactions are less intensive than those of pure titanium (CpTi).

The extent of the tissue reactions decrease to the baseline level

and an ongoing repair process is observed in all specimens after

4 weeks. The particles of the composites show a significantly

lower osteolytic area (OA) and higher bone area (BA) than the

CpTi particles in Figure 20. The percentage of OA is defined as

the ratio of the area of inflammatory granulation tissue to the

total cross sectional area of the calvaria, and the percentage of

BA is defined as the ratio of the area of osseous tissue,

including trabecular bone, to the cross sectional area of the

calvaria. There is no significant difference in the OA or BA for

the two types of composites. This study reveals that wear

particles of the novel LTD-free Zr/Al composite induce a lower

biological response than those of CpTi.

Figure 19. Histological analysis at 1 week after surgery showed moderate biological responses to zirconia/alumina composites butmuch lower than those of titanium, where Z/A#1, Z/A#2, and. Z/A#3 correspond to (5.3Y, 4.6Nb)-TZP/80vol% Al

2O

3, (5.3Y,4.6Nb)-

TZP/20vol% Al2O

3, and (3.0Y, 1.6Nb, 3.6Ce)-TZP/20vol% Al

2O

3, respectively.

Figure 20. Osteolytic area and Bone area after 1 week and 4 weeks.

Figure 21. No phase transformation of zirconia in (a) 20Zr/80Al and (b) 80Zr/20Al composites was observed after sterilization (eth-ylene oxide gas, 25-kGy, and 50-kGy gamma radiation group).


Vol. 15, No. 3

Sterilization of LTD-free zirconia/aluminacomposites

Sterilization by gamma irradiation, particularly at a dose of

25 kGy, is widely used for medical devices and supplies. This

study showed that ethylene oxide gas and 25-kGy gamma irra-

diation are only effective in the sterilization of the surface of the

80Zr/20Al and 20Zr/80Al composites, whereas 50-kGy gamma

irradiation was effective in sterilizing not only the surface, but

also deep inside. The bacteria which was identified in the deep

culture of the ethylene oxide gas and 25 kGy irradiation group

was mainly Bacillus, characterized by high resistance to heat

and other severe environments. It is likely that the bacteria sur-

vived the preparation process of the composite specimens.

Although the zirconia/alumina composites have excellent wear

resistance, the deep inner portion of these composites can be

exposed to external circumstances when excessive wear or

breakage occurs. Moreover, there was no change in the tetrag-

onal phase stability of zirconia and in biaxial strength in the zir-

conia/alumina composites (Figure 21) after the gamma irra-

diation up to 50 kGy. These results suggest that 50-kGy gamma

irradiation is a more optimal sterilization method for zirconia/

alumina composites in total hip replacements.

Acknowledgement

The contribution of D-J Kim was supported by the Small &

Medium Business Technology Innovation Program administrated

by the The Small & Medium Business Administration under

grant 20100490.

References

1. J. Briggs, “Engineering Ceramics in Europe and The USA,” Ence-ram, UK, 1999.

2. FDA, “Recall of Zirconia Ceramic Femoral Heads for Hip Implants,”Sept. 13, 2001.

3. J. Chevalier, “What future for zirconia as a biomaterial?,” Biom-aterials. 27, 535-543 (2006).

4. K. Haraguchi, N. Sugano, T. Nishii, H. Miki, K. Oka, and H.Yoshikawa, “Phase transformation of a zirconia ceramic head aftertotal hip arthroplasty,” J. Bone Joint Surg. 83B, 996-1000 (2001).

5. W. Zhu and X. Zhang, “Aging behavior of tetragonal zirconiapolycrystal (TZP) ceramics in the temperature range of 200oC to350oC in air,” Scr. Mater. 40, 1229-1233 (1999).

6. D-J. Kim, H. Jung, J. Jang, and H. Lee, “Fracture toughness, ionicconductivity, and low-temperature phase stability of tetragonalzirconia codoped with yttria and niobium oxide,” J. Am. Ceram.Soc. 81, 2309-2314 (1998).

7. S. Lawson, “Review: Environmental degradation of zirconiaceramics,” J. Eur. Ceram. Soc. 15, 485-502 (1995).

8. Hummer, C., R. Rothman, and W. Hozack, “Catastrophic failureof modular Zirconia-Ceramic femoral head components aftertotal hip arthroplasty,” J. Arthroplasty 10, 848-850 (1995).

9. D-J. Kim, H. Jung, and D. Cho, “Phase transformations of Y2O3

and Nb2O5 doped tetragonal zirconia during low temperatureaging in air,” Solid State Ionics. 80, 67-73 (1995).

10. D-J. Kim, “Influence of aging environment on low-temperature deg-radation of tetragonal zirconia alloys,” J. Eur. Ceram. Soc. 17, 897-903 (1997).

11. D. Lee, J. Jang, and D-J. Kim, “Raman spectral characterization ofexisting phases in the ZrO2-Y2O3 Nb2O5 system,” Ceram. Int. 27,291-298 (2001).

12. D. Lee, D-J. Kim, and B. Kim, “Influence of alumina particle sizeon fracture toughness of (Y,Nb) TZP/Al2O3 composites,” J. Eur.Ceram. Soc. 22, 2173-2179 (2002).

13. D-J. Kim, “Effect of Ta2O5, Nb2O5, and HfO2 alloying on thetransformability of Y2O3-stabilized tetragonal ZrO2,” J. Am. Ceram.Soc. 73, 115-120 (1990).

14. D-J. Kim, D. Lee, and J. Han, “Low temperature stability of zirconia/alumina hip joint heads,” Key Eng. Mater. 240-242, 831-834 (2003).

15. J. Chevalier, M. Saadaiui, C. Olagnon, and G. Fantozzi, “Double-tor-sion testing of 3Y-TZP ceramic,” Ceram. Int. 22, 171-177 (1996).

16. D-J. Kim, “Effect of Ta2O5, Nb2O5, and HfO2 alloying on the trans-formability of Y2O3-stabilized tetragonal ZrO2,” J. Am. Ceram. Soc.73, 115-120 (1990).

17. D-J. Kim, M. Lee, D. Lee, and J. Han, “Mechanical properties,phase stability, and biocompatibility of (Y,Nb)-TZP/Al2O3 compositeabutments for dental implants,” J. Biomed. Mater. Res. (Appl. Biom-ater.) 53, 438-443 (2000).

18. D-J. Kim, I. Park, K. Lee, J. Chaevalier, H. Attaoui, and J. Han, “Sub-critical crack growth Resistance of low temperature degradation-freezirconia/alumina composites for medical applications,” Key Eng.Mater. 284-286, 1007-1010 (2005).

19. De Aza, A.H., Chevalier, J., Fantozzi, G., Schehl, M, and Torre-cillas, R, Biomater, “Crack growth resistance of alumina, zirconiaand zirconia toughened alumina ceramics for joint prostheses,”Biomater. 23, 937-945 (2002).

20. H. Ko, J. Han, M. Bächle, J. Jang, S. Shin, and D-J. Kim, “Initialosteoblast-like cell response to pure titanim and zirconia/alu-mina ceramics,” Dental Mater. 23, 1349-1355 (2007).

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