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.
108 Dae-Joon Kim, Kwon-Yong Lee, Hee-Joong Kim, and Jung-Suk Han
Biomaterials Research 2011
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.
Feasibility Study of Novel Low Temperature Degradation-free Zirconia/Alumina Composites for Total HIP Replacement 109
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.
110 Dae-Joon Kim, Kwon-Yong Lee, Hee-Joong Kim, and Jung-Suk Han
Biomaterials Research 2011
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.
Feasibility Study of Novel Low Temperature Degradation-free Zirconia/Alumina Composites for Total HIP Replacement 111
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.
112 Dae-Joon Kim, Kwon-Yong Lee, Hee-Joong Kim, and Jung-Suk Han
Biomaterials Research 2011
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).
Feasibility Study of Novel Low Temperature Degradation-free Zirconia/Alumina Composites for Total HIP Replacement 113
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.
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