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



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).

146 TSUKAMOTO ET AL.

Journal of Biomedical Materials Research Part B: Applied Biomaterials



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.





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.

149Zr WEAR AND PHASE-TRANSFORMATION




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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