comparison of cobalt chromium, ceramic and pyrocarbon hemiprostheses in a rabbit model: ceramic...

Comparison of Cobalt Chromium, Ceramic and PyrocarbonHemiprostheses in a Rabbit Model: Ceramic Leads to MoreCartilage Damage Than Cobalt Chromium

Martin Jung,1 Peter Wieloch,1 Helga Lorenz,1 Tobias Gotterbarm,2 Katrin Veyel,1 Mark Daniels,1

Abdul Kader Martini,1 Wolfgang Daecke3

1 Department of Orthopedic Surgery, University of Heidelberg, Heidelberg, Germany

2 Department of Orthopedic Surgery, Brigham and Women’s Hospital, Harvard Medical School, Boston, Massachusetts

3 Department of Trauma, Hand- and Reconstructive Surgery, J.W. Goethe University Frankfurt, Frankfurt a.M., Germany

Received 11 April 2006; revised 24 June 2007; accepted 26 July 2007Published online 31 October 2007 in Wiley InterScience (www.interscience.wiley.com). DOI: 10.1002/jbm.b.30961

Abstract: Cartilage wear after hemiarthroplasty remains a problem in orthopedic surgery.

The main cause of cartilage wear, apart from incongruency of the joint partners, is generally

considered to be the tribology of the material surfaces. This study evaluates in 27 rabbits the

degree of cartilage wear of the tibia plateau after hemiarthroplasty with proximal

interphalangeal prostheses made of three different materials [cobalt chromium (CoCr),

pyrocarbon (PyCa), and ceramic (Cer)]. Three months after hemiarthroplasty, the

articulating tibial cartilage was histomorphologically examined and degenerative damage

was graded using the modified Mankin score. The mechanical capacity of the cartilage was

assessed by stress relaxation testing. The biomechanical properties of the cartilage were

significantly superior in the CoCr group as compared with the Cer group (p < 0.03), indicating

less damage to the articulating cartilage surface. The Mankin score showed significantly lower

values in the CoCr compared with Cer group (p 5 0.011), whereas no differences were found

between PyCa and CoCr or PyCa and Cer. In contrast to earlier reports, in this

hemiarthroplasty model, the CoCr alloy showed less cartilage damage than a ceramic surface.

Further, in vivo experiments are necessary to elucidate the controversial issue of the most

suitable material for hemiarthroplasty. ' 2007 Wiley Periodicals, Inc. J Biomed Mater Res Part B: Appl

Biomater 85B: 427–434, 2008

Keywords: hemiarthroplasty; cartilage; cobalt-chromium (alloys); zirconia; pyrocarbon

INTRODUCTION

Hemi-joint replacement has gained importance in the last

decades particularly due to the development of new materi-

als. In the clinical routine, the materials used for hemiar-

throplasty surfaces include the longstanding cobalt–

chromium–molybdenum alloy, various ceramics such as

zirconium (ZrO2) or aluminium oxide (Al2O3), and pyro-

carbon implants. The latter are preferably used in arthro-

plasty of smaller joints of the hand and foot. Theoretically,

the most suitable material for hemi-joint replacement would

be a material with the lowest possible coefficient of friction

to guarantee the lowest rate of damage to the corresponding

articular surfaces. Up to now only a few experimental stud-

ies have been published concerning cartilage wear in vari-

ous hemiprosthetic material combinations.1–3 Most of the

published data focus on hemiprostheses in hip arthroplasty

and have encouraged clinicians to apply these techniques to

replace damaged articular surfaces in smaller joints (e.g.,

the shoulder, the distal ulnar head, and the proximal pole

of the scaphoid or capitate). Surprisingly, there is no pub-

lished data available addressing the impact and effect of

hemiarthroplasty in smaller joints. In this study, we adopted

the arthroplasty model in rabbit knee joints, originally

described by Minawikawa et al.,4,5 to perform the implanta-

tion of a hemiprosthese and expose the implant to in vivoconditions including load-bearing and shear forces.

In this experiment, we compared three different surface

materials of finger hemiprostheses in the rabbit knee

model: cobalt chromium (CoCr), pyrocarbon (PyCa), and

ceramic (Cer). To our knowledge, no experimental or clini-

cal study comparing these three prostheses materials has

been conducted so far.

Correspondence to: W. Daecke (e-mail: [email protected])Contract grant sponsor: Research Fund of the Department of Orthopaedic sur-

gery, University of Heidelberg

� 2007 Wiley Periodicals, Inc.

427

MATERIALS AND METHODS

Implants

In this study, we used the proximal component of three dif-

ferent proximal interphalangeal prostheses as a hemipros-

thesis. The first prosthesis consisted of a cobalt chromium

alloy-articulating surface at the cap and a Ti-6A1-4V stem

with a pure titanium-sintered coating (CoCr, n 5 9,

Avanta, San Diego, USA, size 3). The second implant was

made of a POCO graphite core on which a 0.5 mm layer of

low-temperature isotropic pyrolytic carbon was deposited

(PyCa, n 5 9, Ascension, USA, size 30P). The core of the

third prosthesis (Cer, n 5 9, Moje, Petersberg, Germany)

was ceramic (ZrO2) with a hydroxyapatite-coated glass ce-

ramic surface (Bioverit I) at the stem. The third prosthesis

was manufactured especially for this experiment and was

designed almost identical to the two above mentioned pros-

theses in the cartilage contact area. The condyle radius var-

ied minimally in the sagittal plane [CoCr: 8.53 mm, Cer:

8.77 mm, and PyCa: 8.88 mm, see Figure 1(A)] and the dis-

tance between the two condyle contact areas in the frontal

plane was the same [6.54 mm, see Figure 1(B)]. The differ-

ences in the cap design as seen in Figures 1(B) and 2 con-

cerned mainly the intercondylar region which did not come

in contact with the opposing tibial cartilage (maximum dis-

tance of 0.26 mm between CoCr and Cer). Differences in

the condylar region displayed a maximum of 0.17 mm and

were negligible compared with the normal interindividual

implantation differences which were in the region of about

1 mm. Further differences were in the undercup region: the

undercup of the Cer prostheses was plane, whereas the CoCr

and PyCa included an angulation of 308 in the posterior half

of the undercup. Again, this difference did not affect the

contact area of the opposing cartilage.

The stem of each implant exceeded the press-fit anchor-

age zone of the 5 mm subcondylar cancellous bone stock.

Surgical Procedures

All animal care, housing, and treatment were performed

according to the German animal welfare act after receiving

written permission for the experiment by the Animal Rights

Protection Authority of Baden Wurttemberg, Germany (Re-

gional Council Karlsruhe, AZ 35-9185.81/G-47/03).

After randomization to one of the three study groups,

the different prostheses were placed into the right distal fe-

mur of 27 skeletally mature New Zealand White rabbits

(Charles River, Kisslegg, Germany) with a minimum age

of 6 months and an average weight of 3.97 kg (SD: 0.44).

There were seen no significant differences of weight or

size of the distal femur between the three study groups.

The surgical procedure was performed as published else-

where6: Under general anesthesia, the skin incision at the

right knee was followed by a medial arthrotomy and the

patella was displaced laterally. Both femur condyles were

resected with a 5 mm osteotome according to the shape of

the undercup of the implant. The anterior cruciate ligament

(ACL) had to be cut to open the medullary cavity. Utilizing

undersized rasps, a press-fit anchorage was reached for all

three prostheses. The implantation was carried out in a

flexed position of 308 in relation to the femur shaft to

achieve an optimal range of motion and to decrease shear

forces on the prosthesis. Closure of capsule and skin was

performed in separate layers with single resorbable sutures

(vicryl 3-0, Ethicon, Norderstedt, Germany). Postoperative

passive range of movement (ROM) was assessed for each

knee after wound closure. Analgesia was obtained by intra-

muscular injection of buprenorphine every 12 h for 5 days

(0.05 mg/kg body weight, buprenorphine, Boehringer,

Mannheim, Germany). A custom-made splint in 908 knee

flexion was applied for 5 days after surgery. The left knee

Figure 1. Exact dimensions (mm) of the cap area of the three pros-

theses and measurement of differences. A: Sagittal plane. B: Frontalplane in the dorsal aspect of the prosthesis.

Figure 2. The three different proximal interphalangeal prostheseswhich were used as hemiprostheses with an almost identical shape

at the cartilage contact areas. A: Cobalt chromium; B: Pyrocarbon;

C: Zirconium oxide.

428 JUNG ET AL.

Journal of Biomedical Materials Research Part B: Applied Biomaterials

served as a nonoperated control. The animals were kept in

standard boxes (64 3 64 3 60 cm3) for 6 weeks. For a fur-

ther 6 weeks, the rabbits were kept in large boxes (250 380 3 220 cm3) to allow full weight bearing under appro-

priate mobility. Two observers carefully examined the ani-

mals for limping during and at the end of the study.

After sacrificing the animals 12 weeks postoperatively

by using a pentobarbital overdose (Narcoren, Merial, Hall-

bergmoos, Germany), passive ROM was again evaluated

for each knee. Afterwards, both tibiae were explanted and

cleaved from adjacent soft tissue and subsequently photo-

graphed.

Biomechanical Measurement

Knees were kept moist with physiological saline until and

during measurement of the biomechanical properties. The

tibiae were fixed in a custom-made indentation testing

machine (Z005/TN25, ZWICK, August-Nagel-Str. 11, 89079

Ulm, Germany).

A standardized stress-relaxation testing protocol de-

scribed by Gotterbarm et al.7 was performed in the center

of the lateral and medial plateau of each tibia.

The thickness of the cartilage was determined at each

measurement area by the needle penetration method de-

scribed by Hoch et al.8 Axial indentation was then carried

out with a plane nonporous steel indenter, 2.0 mm in diam-

eter, attached to a 10 N load-cell (accuracy 0.01 mN). A

preload of 0.1 mN was used to ensure contact with the tis-

sue surface. A compressive strain (50 mm/s) of 20% carti-

lage thickness was then applied. The axial reaction force

was recorded over a time period of 300 s every 0.1 s. Par-

ticular attention was given to the maximum reaction force

and the reaction force after 300 s in mN. Further statistics

were carried out with mean values of the medial and lateral

tibial plateau of each animal.

Histological Scoring

After biomechanical measurement, the tibiae were cut sag-

ittally in the center of the medial and lateral tibia plateau

using a diamond saw. Subsequently, the medial and lateral

specimens were fixed in 4% phosphate-buffered formalin

for 5 days. The formalin-fixed specimens were decalcified

in EDTA for 4–6 weeks, dehydrated, embedded in paraffin,

and serially sectioned (5 lm). Sections were histochemi-

cally stained with a standard protocol for Safranin O/Fast

green (SOFG).

Degenerative alterations of the articular cartilage of the

lateral and medial tibia plateau were quantified by using

the modified Mankin-Score9,10 by two blinded observers

(HL and MJ). Normal articular cartilage scored at 0 points,

worst osteoarthritic changes at 32 points. For further statis-

tical analysis, the mean of the medial and lateral tibia pla-

teau score values of each animal was used.

Statistical Analysis

Biomechanical data is shown in a graph as a function of

axial reaction force over time and in terms of box-and-

whisper plots for maximum reaction force and equilibrium

reaction force at 300 s. Differences between the means of

the three treatment groups were calculated for maximum

force, equilibrium reaction force at 300 s, and additionally

at 1–10, 20, 50, 100, and 180 s, using the nonparametric

Mann Whitney U test.

The mean values of the modified Mankin score and dif-

ferences in ROM after the operation or sacrification of

each hemiprosthese group were also compared with the

Mann Whitney U test.

Association between the modified Mankin score and the

reaction force at equilibrium after 300 s was tested with

the Pearson’s correlation coefficient. For all tests signifi-

cance was reported if p � 0.05.

RESULTS

All animals initially recovered well from surgery. Two ani-

mals had to be excluded from further studies, one due to a

chronic knee infection (Cer group) and another due to a

broken implant (PyCa group). For this reason, the sample

number was reduced to eight in these two groups.

At the end of the study, one animal each of the PyCa

and Cer groups still showed minor limping, whereas all

nine animals of the CoCr group showed normal gait. At

sacrification, all animals showed stability of the operated

knee joint. The ROM of a normal rabbit knee in our series

was 1628 (67.38; mean and standard deviation of three

normal knees). ROM decreased directly after operation in

all operated knees first of all because of the operation pro-

cedure itself by which the prostheses was implanted at 308of flexion in relation to the femur shaft. Second, the limited

arc of motion of the prostheses condyles, which are origi-

nally designed for human proximal interphalangeal joints,

also contributed to the ROM decrease.

The median ROM of the different treatment groups are

shown in a box and whisker plot (Figure 3). The ROMs at

the beginning of the study showed no significant differen-

ces between the groups (CoCr vs. PyCa: p 5 1.0, CoCr vs.

Cer: p 5 0.07, PyCa vs. Cer: p 5 0.1). At the end of the

study, ROM showed no significance between the CoCr and

PyCa groups (p 5 0.4) and the PyCa and Cer groups (p 50.2), whereas the difference between the CoCr and Cer

groups showed a significant difference toward a smaller

ROM in the Cer group with p 5 0.04 (Figure 3).

Biomechanical Properties of Articulating Cartilage

Figure 4 provides an overview of the complete stress-relax-

ation test. All three treatment groups and the left control

knees are shown. Each of the lines resembles the mean of

one treatment group 6 standard deviations. Significant dif-

ferences in reaction force were reached between CoCr and

429CARTILAGE WEAR AFTER HEMIARTHROPLASTY IN AN ANIMAL MODEL


Cer at any subsequent time points (1–10, 20, 50, 100, 180 s,

data not shown), after 300 s (203 6 115 vs. 94 6 115 mN,

p 5 0.029) as well as for mean maximum stress (1240 6419 vs. 435 6 275 mN, p 5 0.002), see Figure 5(A,B).

There was a tendency toward higher test values in the

PyCa group compared to Cer, reaching significance with

p 5 0.05 at maximum reaction force only [Figure 5(A)].

No significant differences of reaction force in the stress-

relaxation test were seen when comparing the CoCr and

PyCa groups. All treatment groups showed significant dif-

ferences of reaction force at all time points compared with

the normal cartilage of the left tibiae.

Histological Evaluation

The evaluation of degenerative cartilage changes with the

modified Mankin score revealed a significant difference in

score values when comparing the CoCr (19.6 6 6.7) and

the Cer (26.6 6 3.1) groups (p 5 0.011). The score values

of CoCr were also superior to PyCa (22.5 6 6.5) but did

not reach significance (p 5 0.28). No significant differen-

ces were seen between the score values of PyCa and Cer

(p 5 0.11). All mean scores of the three prostheses groups

were significantly higher than the control values (3.9 62.1, p\ 0.0001), see Figure 6.

The reaction force at equilibrium after 300 s and the

modified Mankin score showed a significant negative asso-

ciation with a correlation coefficient of r 5 20.73 (p 50.0001).

Qualitative histological examination revealed very simi-

lar changes in the tibial cartilage of all three treatment

groups: deterioration of cartilage integrity started with sur-

face irregularities and slight fibrillation [Figure 7(B)] and

was followed by development of deep clefts throughout all

cartilage layers, accompanied by cell cloning (cluster for-

mation) and a severe loss of glycosaminoclycans of the

extracellular matrix [Figure 7(C)]. Further damage lead to

the loss of chondrocytes in the intermediate and radial zone

and ended with denuded bone in some areas [Figure 7(D)].

Qualitative histological assessment demonstrated a thicken-

ing of the subchondral bone plate after articulating with the

hemiprostheses for 3 months [Figure 8(A,B)] without re-

vealing significant differences when comparing the three

groups (data not shown).

DISCUSSION

Up to the present, little is known about the clinical outcome

and benefit of the various materials used for hemiarthroplasty

in larger and smaller joints; no comparative study on the du-

rability of different hemiprostheses has been published to

date. In a retrospective study on 140 patients treated with a

hemiarthroplasty using a biolox ceramic head for femoral

head fractures, Muller et al. concluded that the advantages of

the ceramic hemiarthroplasty are mainly theoretical and still

have to be proven scientifically.11

The present study was performed to test in vivo degener-

ative cartilage changes in a rabbit hemiprosthese model

using three different surface materials. This rabbit model is

associated with much higher loading forces than those in

human proximal interphalangeal joints: whereas the loading

Figure 3. Box-and-whisker plot showing passive range of motion

(ROM) in degrees directly after operative procedure (white bars) andat the end of the study (grey bars). The first and third quartiles are

displayed as the end of the box, the maximum and minimum as the

whiskers and the median as a vertical bar in the interior of each

box. ROM was significantly decreased at the end of the study in theCer group, compared with CoCr group (#p 5 0.04).

Figure 4. Axial reaction forces of the tibial cartilage after 3 months

as a function of time after stress-relaxation testing. Mean values are

given for each hemiprosthesis group, line bars indicate 6 singlestandard deviation (controls: n 5 26, CrCo: n 5 9, PyCa: n 5 8,

Cer: n 5 8). The first values represent the maximum axial reaction

force after imposing a displacement of 20% of total cartilage thick-ness. The following values represent the time history during the first

10 and at 20, 50, 100, 180, and 300 s. All CoCr values were signifi-

cantly higher compared to the Cer group. All values of the three

prostheses groups were significantly lower than the values of nor-mal control cartilage except the maximum reaction force of CoCr.

430 JUNG ET AL.


force in rabbit knee joints is estimated to be as much as

200 N, the one for PIP joints is about 25 N.12,13 This fact

may overrate the effects of the hemiprostheses on the

opposing articular cartilage in our model compared with

the human situation. Also, the normal relation of PIP flex-

ion force to extension force is divergent in the rabbit knee

joint toward a stronger extension force. Finally, anatomic

structures that are found in the rabbit knee, such as menisci

and cruciate ligaments, are not existent in the PIP joint.

This animal model necessitates a resection of the anterior

cruciate ligament (ACL) to place the prostheses correctly.

The resection of the ACL itself is a procedure which was

used by Pond and Nuki14 in canines and by Vignon et al.15

as an osteoarthritis model in rabbits. When interpreting our

results one must keep in mind that due to the ACL resec-

tion itself some degree of osteoarthritis must be expected

after 3 months, as Pfander described in his study 1999.16

Therefore, the most important finding of this study is not

the occurrence of osteoarthritis per se, but the different

degrees of it in the three hemiprosthesis groups studied.

In absence of other PIP joint animal models, and on the

basis of the limitations mentioned above, this model makes

possible the assessment of principle differences in the out-

come of different hemiprostheses.

Cartilage wear after hemiarthroplasty may be caused by

mechanical factors such as impairment of the surface geome-

try, the contact area between the articulating surfaces and/or

gliding speed.17 The shape of the articulating surfaces may

thus influence cartilage deterioration in a hemiprostheses

model. For that reason, we used three prosthesis models

which presented with an almost identical design of the carti-

lage contact area. This was achieved by manufacturing a

ZiO2 prosthesis which was designed using the pyrocarbon

and chromium cobalt prostheses as models. Minor differences

had to be accepted due to technical reasons: (1) in the inter-

condylar region, which was not as prominent in the ZiO2

prosthesis as in the other two prostheses. This should not

have affected the outcome of this study, since the intercondy-

lar region was not in contact with the opposing cartilage. (2)

In the undercup region of the ceramic prostheses, no angula-

tion could be added, in contrast to the other two prostheses.

Differences in the cartilage contact area were only found in a

dimension of 0.06–0.17 mm. These differences seem to be

negligible keeping in mind the normal differences during the

prostheses implantation process itself (�1 mm).

The surface properties of the hemiprosthese materials

themselves may play a decisive role in cartilage deteriora-

Figure 5. Results of stress relaxation test at maximum reaction (A)

and equilibrium reaction force after 300 s (B) shown in a Box-and-

whisker plot. A: The maximum reaction force shows significant dif-ferences between CoCr and Cer (§p 5 0.002) and PyCa and Cer (p

5 0.05). The Cer and PyCa groups demonstrate significantly lower

reaction force values than the controls (#p \ 0.02). B: The equilib-

rium reaction force after 300 s also shows significant differencesbetween CoCr and Cer (§p 5 0.029). All three prostheses groups

revealed significantly lower values than the controls (#p\ 0.03).

Figure 6. Results of modified Mankin score depicted in a Box-and-

whisker plot. CoCr showed less cartilage damage than Cer (§p 50.001), whereas all prostheses groups showed significantly higherscore values than controls (#p\ 0.0001).



tion due to different friction coefficients and a different

modulus of elasticity.

Pin-on disc trials by Patel and Spector18 demonstrated a

lower coefficient of friction in vitro using zirconium ce-

ramic compared with cobalt chromium. However, in their

study the authors utilized a nonphysiological geometrical

arrangement of the articulating surfaces and the number of

specimen was small (n 5 4). Muller et al. found lower fric-

tion coefficients using a ceramic head (Al2O3) compared

with a metal head (FeCrNiMnMo) in a hip joint simula-

tor.17 Even though the geometrical arrangement in this

study was closer to the normal physiological setting of a

human hip joint, the authors themselves point out that the

measurement of coefficients of friction in a hip joint simu-

lator is not fully comparable with the in vivo situation. In

regard to the friction coefficient measured in vitro, the ce-

ramic prostheses should produce the lowest amount of

destruction at the opposing cartilage; however, this has not

been proven in clinical trials to date. In the present study,

we observed in an in vivo model the lowest cartilage dam-

age using a CoCr alloy, although the friction coefficient of

a cobalt chromium alloy is known to be higher than of a

ZiO2 ceramic.18 This may indicate that there were

more important factors other than the friction coefficient

which played a role in cartilage damage in our in vivoexperiment.

Figure 7. Cartilage changes 12 weeks after hemiarthroplasty implantation. A: Normal cartilage surface.

B: Surface irregularities and slight fibrillation at the upper cartilage layer. C: Fibrillation and deep cleftsto the intermediate zone and a severe loss of proteoglycans in the ectracellular matrix. D: Loss of all

cartilage layers leading to the denudation of the bone. [Color figure can be viewed in the online issue,

which is available at www.interscience.wiley.com.]

Figure 8. A: Normal subchondral bone plate (bracket). B: Thickened subchondral bone plate (bracket)

after articulating for 3 months with a hemiprosthese, indicating adaptation to increased load at the

contact area. [Color figure can be viewed in the online issue, which is available at www.interscience.wiley.com.]

432 JUNG ET AL.


Up to now, animal experiments on hemiarthroplastic

replacement have been reported mainly in hip joints of can-

ines.1–3 Cook et al. found lower levels of gross cartilage

wear, cartilage fibrillation, and glycosaminoglycan loss

using a low temperature isotropic pyrolytic carbon surface

compared with a cobalt-chromium-molybdenum or titanium

alloy.1 They mentioned two possible reasons for that find-

ing. First, they saw extensive bone ingrowth into the po-

rous-coated stem with the two metallic devices, which

consequently could have increased the degree of implant

loading transferred to the joint, whereas in the LTI carbon

implants only direct bone apposition was observed. This

may have lead to a diminished load transmission to the

articulating cartilage. Second, the lower stiffness of the LTI

carbon implant may have decreased the contact pressure

against the cartilage.

In agreement with Cook et al., Daecke et al.6 could

show in their experiment that the PyCa implant/bone inter-

face showed at best only bone apposition, whereas the

CoCr alloy with the porous titanium shaft was very stably

anchored to the femur by bony ingrowth. Looking at the

literature, hydroxyapatite coated ceramics are known to ex-

hibit stable bone ingrowth.19 We did not further assess

stem fixation except for proving secure fixation in the cor-

rect position at sacrification. However, on the basis of the

literature and our histological and biomechanical findings,

we conclude that the more ‘‘dynamic’’ fixation to the bone

did not prevent cartilage damage in the PyCa group unlike

Cook et al. suggested. The groups with theoretically stable

bony ingrowth (Cer and CoCr) showed very different pat-

terns in cartilage damage, underlining that it is also not

only the bony ingrowth of the implant being responsible

for the cartilage damage.

Maistrelli et al. compared a ceramic head versus a

cobalt chromium cephalic component in a canine hemiar-

throplasty model.3 In both groups, the same femoral stem

was implanted using the press fit technique, excluding

divergence due to different fixation procedures. Even

though less pronounced cartilage damage was found after 5

months in the ceramic group, no differences between the

groups were found after 8 months. We cannot rule out that

there were different patterns of cartilage damage during the

3 month period between the three treatment groups of our

study, as we only examined one time point.

Statements about the exact elasticity modulus of thethree materials used are rare in literature. This may be dueto the fact that for example the CoCr prosthesis was a‘‘bicomposite’’ model with a titanium stem and a prosthesiscup of a chromium-cobalt alloy, making it difficult to gaininformation about the elasticity modulus from the literature.For comparison, we used the elasticity modulus as given inliterature for the chromium cobalt alloys, despite the‘‘bicomposite’’ manufacturing of this prosthesis. The elas-ticity modulus of the three prostheses we used was quitedifferent. The lowest was PyCa with 20–25 GPa, whereasthe elasticity modulus for CoCr and Cer prostheses exhibit200–240 GPa and 200 GPa, respectively. Unlike Cook

et al.,1 we did not find significant differences within theCoCr and PyCa group concerning cartilage damage. Never-theless, the elasticity modulus of these two materials dif-fered about 10-fold. The two materials with a similarelasticity modulus, however, revealed very different carti-lage damage (CoCr and Cer).

Limitations of our study are mainly connected with the

animal model: The weakness of this model must be seen in

the different anatomy of the rabbit knee joints compared

with human PIP joints, like mentioned above. A compari-

son of three different prostheses materials should anyhow

be feasible, as long as the cartilage contact areas of the

three prostheses show the same shape. Therefore, the most

important influence for differences in cartilage wear should

be due to the material properties.

Contrary to our expectation, we found the most pro-

nounced cartilage damage in the Cer group whereas the

CoCr alloy showed significantly less cartilage damage. The

histological and biomechanical results of the PyCa prosthe-

ses were between these two prostheses but did not reach

significance.

Theoretical considerations would favor ZrO2 over a

cobalt-chromium hemiprosthesis because of the lower coef-

ficient of friction, whereas the pyrocarbon hemiprosthesis

could be superior due to the low coefficient of friction and

a modulus of elasticity which is closest to the modulus of

elasticity of bone. However, in this in vivo study none of

these material properties played a decisive role in cartilage

damage after 3 months.

After in vivo implantation ZiO2 implants may change

their surface properties in terms of roughening by phase

transformation, as Haraguchi et al. could show.20 Accord-

ing to Santos et al.21 the alterations of ZiO2 by phase trans-

formation does not seem to be the only reason for the

roughening of ZiO2 surfaces. They suggested that other fac-

tors such as third body wear may play an additional role in

the longterm health of ZiO2 surface. In our study, third

body wear is not likely to affect the results, since no addi-

tional materials were used. Therefore, surface roughening

by phase transformation may explain the most severe carti-

lage damage in our Cer group.

Further experimental trials and prospective clinical

studies are necessary to gain more insight into the clini-

cally important issue of the most suitable hemiprosthesis

materials.

The authors thank R. Fohr and K. Goetzke for their expertisein preparing the histological sections.

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comparison of cobalt chromium, ceramic and pyrocarbon hemiprostheses in a rabbit model: ceramic...

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