ytzp vs uhmwpe
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Y-TZP Zirconia Run Against Highly Crosslinked UHMWPE TibialInserts: Knee Simulator Wear and Phase-Transformation Studies
Riichiro Tsukamoto,1 Paul A. Williams,1 Ian C. Clarke,1 Giuseppe Pezzotti,2 Hiromu Shoji,1
Masao Akagi,3 Kengo Yamamoto4
1
Department Joint Research Center, Peterson Tribology Laboratory, Loma Linda University and Medical Center,Loma Linda, California
2 Ceramic Physics Laboratory, Kyoto Institute of Technology, Kyoto, Japan
3 Department of Orthopaedic Surgery, Kinki University, Osaka, Japan
4 Department of Orthopaedics, Tokyo Medical University, Tokyo, Japan
Received 16 April 2007; revised 4 August 2007; accepted 24 September 2007 Published online 27 December 2007 in Wiley InterScience (www.interscience.wiley.com). DOI: 10.1002/jbm.b.30999
Abstract: Background: Zirconia (ZrO2) ceramics combined with highly cross-linkedpolyethylene appears to be a promising approach to minimize wear in artificial knee joints.
The wear performance of yttria-stabilized zirconia (YZr) femoral condyles on 7-Mrad tibial
inserts was compared in a knee simulator to CoCr bearing on 3.5-Mrad inserts. Methods: The
knee design was the Bi-SurfaceTM type with a 9-year clinical history in Japan (JMM, Japan).
A displacement-controlled knee simulator was used with kinematics that included 20 8 flexion,
658 rotation, and 6 mm anterior/posterior translation. Lubricant was alpha-calf serum, test
duration was 10 million cycles (10 Mc), and wear was measured by weight-loss techniques. The
wear zones were studied by laser interferometry, scanning electron microscopy, and Raman
microprobe spectroscopy. Results: At 10 Mc the wear rates of the CoCr controls averaged
4.5 mm3/Mc. This was within 7% of the prior estimate at 5-Mc duration and comparable to Bi-
Surface wear data from another laboratory. The CoCr condyles increased in roughness ( Ra)
from <50 nm to average Ra5 250 nm due to linear scratching. The ceramic condyles remained
pristine throughout the wear study ( Ra <7nm). With the YZr/7-Mrad combination, the weight
change had a positive slope over at 10 Mc, which meant that the actual polyethylene wear wasunmeasurable. Microscopic examinations at 10 Mc showed that the zirconia surfaces were
intact and there was no detectable change from tetragonal to monoclinic phase. Interpretation:
Our laboratory knee wear simulation appeared very supportive of the 9-year YZr/PE clinical
results with Bi-Surface total knee replacements in Japan. ' 2007 Wiley Periodicals, Inc. J Biomed
Mater Res Part B: Appl Biomater 86B: 145–153, 2008
Keywords: zirconia; polyethylene; knee prosthesis; wear
INTRODUCTION
Total knee replacements (TKR) have shown a 95% survival
rate out to 15 years.1,2 However, contemporary high-risk
cases include much younger and potentially more active
patients with much increased life expectancy. Contempo-
rary TKR enhancements include both highly cross-linked
polyethylenes (HXPE) for tibial inserts and zirconia ce-
ramic for femoral condyles3–7 (Table I).
Zirconia was introduced as a high-strength and high-
toughness alternative ceramic to alumina.8 Such zirconia
designs could be manufactured with virtually identical
dimensions to a CoCr condylar design. The two main zir-conia ceramics considered for use in medical implants over
the past 20 years have been the magnesia and yttria-stabi-
lized types. Of these the yttria-stabilized ‘‘tetragonal zirco-
nia polycrystal’’ (Y-TZP) type has seen the majority of
development and use.9–12 Zirconia has a crystallographic
metastability at body temperature, manifested by a reversi-
ble transformation between two of its main polymorphs,
the tetragonal and the monoclinic phase (t?m transforma-
tion). Significant strengthening can be induced via this
t?m transformation. A volume increase of 3–4% is pro-
duced upon transformation that hinders the progression of
Correspondence to: I. C. Clarke (e-mail: [email protected])Contract grant sponsors: Peterson Family Foundation, Loma Linda University,
Japan Medical Materials Corporation, the Western Center for Orthopaedics Research,
the Tsukamoto Orthopedic Clinic, the Mine Dental Clinic, and the Department of
Orthopaedics, Loma Linda University Medical Center
' 2007 Wiley Periodicals, Inc.
145
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any advancing fatigue crack.10 However, similar changes
on the implant’s bearing surfaces may produce grain pro-
trusion and pull-out with significant impact on surfaceroughening.11,13 This has the potential to accelerate the
wear of mating UHMWPE bearings.
For total knee joint replacements (TKR), the yttria-stabi-
lized zirconia (YZr) has been used clinically in since 1989
in Japan.14 The magnesia-stabilized zirconia (Mzr) has also
been developed for femoral condyles.15 However, the only
ceramic knee with FDA approval to market incorporates a
thin zirconia surface overlying metallic femoral condyles16
manufactured from zirconium (OxiniumTM, Smith and
Nephew, Memphis, TN).
Laboratory studies consistently reported superior wear
resistance for zirconia/UHMWPE combinations when com-
pared with controls.17–20 Such simulator studies did not
comment on any surface roughening after 5 million cycles
duration. Additional scanning electron microscopy, X-ray
diffraction, and Raman spectroscopic analysis would have
been necessary to determine if any phase transformation
had occurred in vitro, as has been reported in a small num-
ber of zirconia balls retrieved ex vivo.13,21–24
One simulator study25 revealed that ZrPE bearings ran
at more than double the surface temperature of MPE bear-
ings. Such temperatures likely resulted in considerably
increased precipitation rate of serum proteins.26 Therefore,
it would appear that the increased thermal insulating prop-
erties of the zirconia resulted in higher lubricant tempera-tures, greater serum degradation and such an artifact led to
a reduction in Zr/PE wear.12 However by 1999, one unfor-
tunate manufacturing change by a French vendor brought
zirconia ball sales to a sudden halt. This change in sinter-
ing processes resulted in a suddenly large number of frac-
tured Prozyr TM balls (St Gobain Desmarquest, France).
This initiated a world-wide recall in 1999 that was con-
cluded by 2001.11,27–29
The performance of Zr/PE combinations has been stud-
ied in knee simulators (Table I). As in the hip joints, all
TKR wear studies indicated superior performance for Zr/
PE when compared with controls. However, once again,
none appeared to have studied the zirconia bearings by mi-
croscopic techniques on conclusion of wear studies. Theauthors previously reported on a TKR wear study3 of a
zirconia knee design that has a 9-year clinical history in
Japan.14 This compared wear with combinations of zirconia
and CoCr femoral condyles matched with 3.5 and 7 Mrad
irradiated tibial inserts. As in all prior Zr/PE wear studies,
the zirconia combinations showed wear reductions when
compared with controls.
Thus up to this point, no laboratory study had looked
for microscopic changes in the zirconia femoral heads or
femoral condyles. We, therefore, elected to extend our prior
knee wear study3 to 10 million cycles (Mc) duration for
combinations Zr/7-Mrad and control CoCr/3.5-Mrad combi-
nations to determine if we could (i) better quantify such
wear performance and (ii) detect any tetragonal to mono-
clinic phase transformation in zirconia femoral condyles.
METHODS
This study was a 5.5–10 Mc extension of our previous
knee simulator report presented at 5.5 Mc duration3 using
the Bi-SurfaceTM knee design (Japan Medical Materials
JMM Inc, Osaka, Japan) (Figures 1 and 2). We studied the
ZrO2 and CoCr femoral condyles using the posterior-cruci-
ate sacrificing configuration.
3
The tibial inserts (n5
6)were made from one lot of ram-extruded PE.3 Gamma-
irradiation sterilization to 3.5-Mrad and 7-Mrad was per-
formed under nitrogen atmosphere. Post-irradiated annealing
was conducted at 1308C for 4 h. Shelf aging averaged
24 months from sterilization to initiation of the wear study.
Additional tibial inserts were used as soak controls.
Knee joints provide an additional level of complexity when
compensating for fluid absorption. Because of the antero-
posterior tracking of the small femoral contact area along
the tibial surface, there will be alternating effects of
motion, loading, and some frictional heating that cannot be
TABLE I. Summary of TKR Wear Simulation Studies
Study Dosage (Mrad) Femoral Condyle Simulator Wear Rates (mm3 /Mc) Ratio (%)
Akagi et al.7
Non CoCr SWM 11.80 ref
3.5 CoCr 6.40 54
7 CoCr 2.90 25
Ueno et al.4
3 CoCr AMTI 3.44 ref
3 Alumina 0.67 19
3 Zr 0.71 21Ezzet et al.
5Non CoCr AMTI 21.30 ref
Non OxZr 12.40 58
Tsukamoto et al.3
3.5 CoCr SWM 4.86 ref
3.5 Zr 1.79 37
7 CoCr 1.59 39
7 Zr Weight-gain N/A
Zr, Zirconia; OxZr, Oxidized zirconium; N/A, not available; AMTI, 6 station knee simulator (Boston, Watertown, MA); SWM, 6 station knee simulator (Shore-Western
Manufacturing, Monrovia, CA).
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were collected with a triple monochromator spectrometer
with wavelength 488 nm (T-64000, ISA, Jovin-Ivon/Horiba
Group, Tokyo, Japan) equipped with a charge-coupled de-
tector (high-resolution CCD camera). A confocal configura-
tion for the Raman probe was selected by placing a pinhole
aperture with diameter 100 nm in the optical train of the
spectrometer and using it to regulate the rejection of out-
of-focus light.42 All Raman measurements were made with
placing the focal plane of the probe on the sample surface.
According to this probe configuration, the probe depth
below the sample surface was experimentally determined
as 6.5 6 0.2 lm, according to defocus probe response
function measurements.42 The volume fraction of mono-
clinic polymorph, V m, in partially transformed zones can be
quantitatively evaluated from the relative intensities of
selected Raman bands belonging to the tetragonal (150 cm21
band) and to the monoclinic (180 and 190 cm21 bands) poly-
morphs, according to the following equation43:
V m ¼0:5ð I 180 þ I 190Þ
0:5ð I 180 þ I 190Þ þ 2:2 I 150
ð1Þ
where I is the integral intensity of a Raman band whose
maximum is located at the subscript wavenumber. In this
study, each data point represented the average of 625Raman measurements on the implant surface. The tech-
nique was judged accurate to about 0.2% by volume. The
precision of the Raman measurement is dictated by the
spectroscopic detection limit of the monoclinic doublet
located at 180 and 190 cm21. The confocal Raman Spec-
trometer was used to provide precise mapping of the
transformation process. The zirconia femoral condyles
were entirely scanned with the confocal microprobe and
the Raman spectra belonging to their surface recorded and
analyzed.22,44–46
The worn and non-worn surfaces of CoCr and zirconia
femoral condyles were analyzed after 10 Mc duration by
laser interferometry (NewView 600, Zygo, Sunnyvale, CA).
The wear patterns were analyzed and compared with the
examination by scanning electron microscopy (LV-SEM:
Philips, XL30-FEG).
Linear regression analysis was performed for each speci-
men using the SPSS statistical package (version 10: SPSS,
Chicago, IL). One-way ANOVA and non-parametric
(Kruskal-Wallis) tests were performed with subsequent test-
ing for multiple comparisons (critical p-value 5 0.05).
RESULTS
The wear trends appeared stable from 1 to 10 Mc duration.
The control implants (CoCr/3.5-Mrad) had excellent linear
trending with respect to test duration (regression coefficient
r [ 0.99), showed good control of experimental variance
(\610% with weight-loss averaging 24.26 mg/Mc and
corresponding wear rates 4.54 mm3 /Mc [Figure 3(a), Table
II]. The ZrO2 /7-Mrad combination showed equally good
linear trending (r [ 0.99) and also excellent control of
experimental variance (\65%). However, the net-weight
change remained positive throughout the study (average
6 0.54 mg/Mc). This meant there was no quantitative
Figure 3. (a) Excellent linear trending (weight-gain) evident for 3.5-
Mrad PE/CoCr combination ( N 5 3) from 1 to 10 Mc duration, (b)
Excellent linear trending (weight-gain) evident for 7-Mrad PE/zirconia
combination ( N 5 3) from 1 to 10 Mc duration.
TABLE I I. Summary of the Experimental Results of Wear Testing
Femoral
Condyle
Dosage
(Mrad) Specimens r -Value
Weight Loss
(mg/Mc)
Wear Rate
(mm3 /Mc)
Avg Wear
Rate (mm3 /Mc) SD SD%
CoCr 3.5 KL271 20.995 24.75 25.06 24.54 0.43 9.60%
CoCr 3.5 KL272 20.998 23.92 24.18
CoCr 3.5 KL273 20.992 24.11 24.38
Zirconia 7 KL280 0.990 0.52 NA NA 0.02 4.60%
Zirconia 7 KL281 0.993 0.53
Zirconia 7 KL282 0.994 0.57
6SD, standard deviation; NA, not available.
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assessment for ZrO2 /7-Mrad wear trends for 10 Mc dura-
tion [Figure 3(b), Table II]. Under SEM study the non-
worn surfaces of CoCr condyles revealed faint polishing
scratches with many carbide formations present. The worn
areas of the CoCr femoral condyles examined at very lowmagnifications (253), revealed numerous deep scratches
orientated in the antero-posterior direction [Figure 4(a)].
These were up to 1 lm wide and hundreds of microns
deep. In contrast, both worn and non-worn areas on the zir-
conia condyles appeared featureless up to a magnification
of 10003 [Figure 5(a)]. The original polishing tracks first
became visible as very fine linear striations [Figure 5(a)].
At mid-level magnifications, parallel arrays of surface
scratches were evident and orientated predominantly in the
antero-posterior direction [Figure 4(b,c)]. At this level the
zirconia surface appeared to have a dimpled appearance. At
higher magnifications [Figure 5(b)], the dimples corre-
sponded to the diameter of the zirconia grains (0.3 lm).Such zirconia surfaces appeared intact with no evidence of
individual grain erosion or larger voids.
Laser interferometry comparisons of worn and non-worn
CoCr surfaces revealed considerable increase in condylar
roughness. Non-worn CoCr bearing surfaces ranged 30–50 nm
range whereas the worn surfaces ranged 110–420 nm (Table
III: Ra). This represented a 6-fold increase in CoCr condylar
roughness over the 10-Mc duration of the simulator study. In
Figure 4. Scanning electron microscopy of the CoCr femoral condyle after 10 Mc; (a) 5 325, (b) 5
31000, and (c) 5 35000.
Figure 5. Scanning electron microscopy of the zirconia femoral condyle after 10 Mc; (a) 5 31000
and (b) 5 320,000.
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unchanged at 10 Mc duration ( Ra \ 8 nm) whereas the
CoCr wear zone had increased from generally an average Ra\ 50 nm to average Ra 5 250 nm (range 100–400 nm)
by end of study. This 5-fold increased roughness was a
result of large, linear, and parallel scratches that developed
in the antero-posterior sliding direction. What was interest-
ing was that these scratched surfaces had no discernable
effect on the overall wear rates of the 3.5-Mrad polyethylene
inserts from 1 to 10 Mc duration. The regression coefficients
averaged[0.99, which represented excellent linear trending
for the control set. It was possible that these scratches devel-
oped early in the study but we had not quantified any early
changes in roughness (from 0 to 1 Mc).
It is also not clear how condylar roughness effects would
accelerate polyethylene wear in vivo.50 Very little has been
reported for CoCr roughness effects in TKR designs, either in
vitro or in vivo. What our study does show was that the CoCr
condylar surfaces were susceptible to 3rd-body abrasion in a
presumed pristine laboratory test. The most likely source for 3rd-body abrasive particles would be either the carbides
released from the CoCr bearing surfaces as a result of the wear
process or polymethylmethacrylate particles released by
cement degradation under the femoral condyles.
This appears to be the first examination of Y-TZP zirco-
nia bearings for phase transformation in a simulator wear
model.12,23,44,45 At 10 million cycles duration, microscopic
examination by a highly spatially-resolved, confocal Raman
probe did not detect any transformation on the surfaces of
the zirconia condyles. In addition the SEM study showed a
pristine surface with rounded sub-micron zirconia grains
visible. There was no surface dimpling or grain eruption
that might have signified that the tetragonal to monoclinic
phase transformation had initiated.12,51,52 The roughness
profiles were also reassuring in that the various surface
parameters showed no change over 10-Mc duration.
On the other hand, a prior review of the literature
showed that no hip simulator study had observed such
transformation effects in vitro and yet such changes clearly
had occurred in the various retrieval studies of Y-TZP zir-
conia femoral heads.12,51 Therefore, it is not known how
predictive our knee simulator data will be for in vivo per-
formance of the zirconia femoral condyles. Our laboratory
has not had the opportunity to analyze any retrieved zirco-
nia knee joints. However, since such Y-TZP zirconia kneeimplants have been used clinically for about a decade14
there may also be an expectation that the transformation
risks are reduced in TKR designs. The use of alumina-doped
zirconia11 may lead to a superior stability as compared to the
non-doped zirconia used previously.12 It is also possible that
the hydrothermal wear effects in zirconia knee implants may
be totally different from that present between the zirconia
femoral heads and polyethylene acetabular liners.
The authors thank Donaldson Arthritis Research Foundation(Colton, CA) for the use of the laser interferometry equipment.
Figure 6. Comparison of the non-worn and worn surfaces of
CoCr and zirconia femoral condyles analyzed after 10 Mc by laser
interferometry.
Figure 7. Raman spectroscopic sites (a, b) studied on zirconia fem-
oral condyles. [Color figure can be viewed in the online issue, which
is available at www.interscience.wiley.com.]
Figure 8. Typical Raman spectrum showing only tetragonal phase
after 10 Mc duration.
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