comparison of cobalt chromium, ceramic and pyrocarbon hemiprostheses in a rabbit model: ceramic...
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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
Journal of Biomedical Materials Research Part B: Applied Biomaterials
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.
Journal of Biomedical Materials Research Part B: Applied Biomaterials
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).
431CARTILAGE WEAR AFTER HEMIARTHROPLASTY IN AN ANIMAL MODEL
Journal of Biomedical Materials Research Part B: Applied Biomaterials
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.
Journal of Biomedical Materials Research Part B: Applied Biomaterials
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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