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Caracterizarea a 10 Cimenturi Comerciale
Transcript of Caracterizarea a 10 Cimenturi Comerciale
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Tensile Characteristics of Ten Commercial Acrylic Bone Cements
E. J. Harper, W. Bonfield
IRC in Biomedical Materials, Queen Mary and Westfield College, University of London, Mile End Road, London, E1 4NS,
United Kingdom
Received 23 July 1999; revised 27 March 2000; accepted 27 March 2000
Abstract: The mechanical properties of acrylic bone cement, used in orthopedic surgery, are
very influential in determining successful long-term stability of a prosthesis. A large number
of commercial formulations are available, differing in chemical composition and physical
properties of both powder and monomer constituents. In this study, the static and dynamic
tensile characteristics of a number of the most commonly used bone cements (Palacos R,
SimplexP, CMW1 & 3, Sulfix-60, ZimmerDough), along with some newer formulations
(Endurance, Duracem 3, Osteobond and Boneloc), have been investigated under the same
testing regimes. Testing was performed in air at room temperature. Significant differences in
both static and fatigue properties were found between the various bone cements. Tensile tests
revealed that PalacosR, Sulfix-60, and SimplexP had the highest values of ultimate tensile
strength, closely followed by CMW3, while ZimmerDough cement had the lowest strength.
Fatigue testing was performed under stress control, using sinusoidal loading in tensiontension, with an upper stress level of 22MPa. The two outstanding cements when tested in
these cyclic conditions were Simplex P and Palacos R, with the highest values of Weibull
median cycles to failure. Boneloc bone cement demonstrated the lowest cycles to failure.
While the testing regimes were not designed to replicate exact conditions experienced by the
bone cement mantlein vivo, there was a correlation between these results and clinical outcome.
2000 John Wiley & Sons, Inc. J Biomed Mater Res (Appl Biomater) 53: 605616, 2000
Keywords: acrylic bone cement; cemented arthroplasties; mechanical testing; fatigue; frac-
ture
INTRODUCTION
Bone cement is used as a grout to fix implants in place during
joint replacement surgery. A polymer powder based upon
poly(methylmethacrylate) (PMMA) or a related co-polymer
is mixed in surgery with a monomer, usually methylmethac-
rylate (MMA). Chemical and physical processes occur simul-
taneously, resulting in a doughy mass, which is inserted into
the prepared cavity. The material sets, stabilizing the implant
approximately 1520 min after the initial mixing. In the body,
bone cement is subjected to a repetitive loading pattern.1
Although bone cement is reasonably strong in compression, it
is a relatively brittle material, making it susceptible to frac-
ture as a result of tensile loads. It is not surprising, therefore,
that bone cement has been implicated as one of the factorsthat causes aseptic loosening.2-4 The Swedish National Hip
Registry found aseptic loosening to be the most common
reason for revising a hip replacement, producing 73.2% of all
revisions recorded between 19791996.5
Following the introduction of bone cement by Sir John
Charnley in the 1950s, there have been numerous investiga-tions into its fatigue properties, which have been comprehen-
sively reviewed by Krause and Mathis6 (from 19741987)
and Lewis7 (from 19871997). These reviews demonstrate
the large variety of fatigue protocols that have been followed
and the limited number of cements studied using any given
testing technique. This background makes it almost impossi-
ble to compare results from different investigations. In this
current investigation, the same fatigue testing method has
been used to assess the tensile fatigue behavior of several of
the commercial cements in clinical practice to provide an
independent assessment of their relative fatigue behavior.
Mechanical tests can be chosen to suit particular cement
types, which do not reveal important characteristics of the
material. This is especially important for the introduction of
new cement formulations, since the ISO standard for ortho-
pedic bone cement8 does not at present include a comprehen-
sive mechanical testing program. Due to the cyclic stresses
bone cement is subjected toin vivo, fatigue properties of bone
cement are an important factor in the long-term survival of a
cemented hip replacement. The experimental procedure em-
ployed in this study was not designed to be the same as the
physiological environment of bone cement. However, there
Correspondence to: Dr. E. J. Harper, IRC in Biomedical Materials, Queen Mary
and Westfield College, University of London, Mile End Road, London, E1 4NS, UK
(e-mail: [email protected])
Contract grant sponsor: EPSRC
2000 John Wiley & Sons, Inc.
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was a direct correlation between the results of the fatigue
testing and clinical data reported in the Swedish National Hip
Register.5
MATERIALS AND METHODS
Commercial Cements
The bone cements used in this study included a combination
of established commercial cements and some newer cements
introduced into the market more recently. The cements tested,
and their manufacturers, were Palacos R (E. Merck, Darm-
stadt, Germany), Surgical Simplex P (Howmedica Interna-
tional Ltd., London, UK), CMW Types 1 & 3 and Endur-
ance (Depuy Ltd., Blackpool, UK), Zimmerdough type and
Osteobond copolymer cement (Zimmer, Warsaw, IN), Sul-
fix-60 (Sulzer, Winterthur, Switzerland), Duracem 3 (Sulz-
erMedica, Sulzer Orthopaedics Ltd., Baar, Switzerland) and
Boneloc (Polymers Reconstructive A/S., Farum, Denmark).
Boneloc has now been withdrawn from the market follow-
ing some early loosening of joint prostheses. Although eachcement is mainly based upon a PMMA homopolymer or
MMA copolymer with an MMA monomer, all cements have
a distinctive formulation leading to different handling and
resultant mechanical properties. A list of the components of
each polymer powder and liquid is given in Table I; this
information is taken directly from the manufacturers infor-
mation included in the cement packaging.
Preparation of Cements
All the cements were prepared according to the manufactur-
ers instructions at 23C; this procedure was very similar for
each cement. The polymer powder was placed in a clean glassbeaker, and the monomer was added and stirred using a
spatula until the powder was fully wetted. The time this took
varied with each cement, but was always less than 40 s. The
mixture was subsequently either transferred to the syringe
body of a cement gun and injected into a PTFE mould at
approximately dough time, usually about 2 min, or manually
inserted into the mould, according to the manufacturers
instructions. The filled moulds were pressurized to 1.4 MPa
and held there until the cement had hardened, approximately
15 min. The exception to this procedure was Palacos R,
which was precooled to 4C prior to mixing.
A maximum of eight half-sized ISO 527 multipurpose test
specimens were produced from each 40 g powder sachet. Thesamples were 75 mm in length, 5 mm in width, approximately
3.5 mm in thickness, with a gauge length of 25 mm. A
schematic diagram of the specimen design is shown in Figure
1. Each sample was measured and stored for at least one week
at 37C in dry conditions prior to testing.
Mechanical Testing
Tensile testing was conducted on an Instron Testing Machine,
Model No. 6025 with a clip-on extensometer to measure
specimen extension. The cross-head speed employed was 5
mm/min, and the maximum force recorded was used to obtain
the ultimate tensile strength, ult. A value for secant modulus,
Esec, was taken at 10 MPa and strain at failure, f, was also
calculated. At least five specimens were tested, and the mean
and standard deviation is reported for each cement.
The fatigue tests were performed on an MTS 810 elec-
trohydraulic testing machine and cycled continuously in load
control until failure. The cyclic stress employed was sinusoi-dal at a frequency of 2 Hz. Ten specimens of each cement
were tested in tensiontension with a lower stress of 0.3 MPa
and an upper stress of 22 MPa. All the mechanical testing was
conducted in air at room temperature.
A Weibull model was used to represent the fatigue data
graphically. The fatigue lives were sorted in ascending order
and each datum point was assigned a median rank value, P ,
obtained from a statistical table.9 This value was used to
calculate a Weibull number,W, for each fatigue life using the
following equation:
W
log(1/(1-P))
The Weibull number was plotted against cycles to failure,
using a logarithmic scale, to produce a straight line represen-
tation of each set of fatigue data. A value for Weibull median
was calculated for each cement type at 50% failure probability.
Scanning Electron Microscopy
Electron microscopy was carried out using a JEOL scan-
ning electron microscope. Both tensile and fatigue fracture
surfaces were examined after the application of a gold coating
using an accelerating voltage of 10 kV. Micrographs of
details of interest were taken at a range of magnifications.
RESULTS
Table II shows the results of the tensile testing, giving mean
values for ultimate strength, secant modulus, and strain at
failure. Numbers shown in brackets are the standard devia-
tions. The cements are ranked according to the mean value
obtained for the ultimate strength. Statistical analysis was
used to assess differences between the maximum strengths of
the cements using a studentt-test and the results are displayed
in Table III. The results from the fatigue testing, Weibullmedian, and range of cycles to failure are displayed in Table
IV. These cements have been ranked according to the value
obtained for the median cycles to failure. Again statistical
analysis was performed, applying the MannWhitney U-
test,10 and, in this case, results are shown in Table V. The
distributions of cycles to failure for each cement are shown
on the Weibull plot in Figure 2 (a). Figures 2(b) and (c) show
the same results on a different scale to enable differences in
the distributions to be observed more easily. Scanning elec-
tron micrographs of the various fatigue fracture surfaces are
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TABLE I. Compositions of Commercial Bone Cements Tested
Key:
Polymers:
PMMA-poly(methylmethacrylate)
P(MMA/MA)-methylmethacrylate/methacrylate copolymer
P(MMA/sty)-methylmethacrylate/styrene copolymer
P(MMA/BMA)-methylmethacrylate/butylmethacrylate copolymer
Initiator:
BPO-benzoyl peroxideMonomers:
MMA-methylmethacrylate
BMA-butylmethacrylate
DCMA-n-decyl methacrylate
IBMA-isobornyl methacrylate
Accelerators:
DMT-N,N-dimethyl-p-toluidine
DMPE-N,N-dimethyl-amino-phenethanol
DHPT-Dihydroxyl-propyl-p-toluidine
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shown in Figures 310. Only micrographs exhibiting features
of interest are included.
DISCUSSION
The handling characteristics of each cement varied consider-
ably, thus requiring differing amounts of mixing to fully wet
the powder. Since all mixing in this particular study was
performed in air, this methodology resulted in a varying
degree of porosity among the cements. This observation was
made via a visual examination of fracture surfaces; no de-
tailed porosity measurements were conducted. It was decided
that it was more clinically relevant to include all samples that
were possible to test, in the tensile and fatigue testing, rather
than use any specimen rejection criteria, as has been used in
previous studies.11 Therefore, if failure occurred in the gauge
length, the value was taken to be a valid result. In practice, the
number of specimens resulting in pores in the fracture surface
greater than 2 mm was very low. The majority of pores, if
present, were less than 1 mm in diameter.
Comparison of the Strengths of the Cements
The values from the static tensile testing highlighted the large
distribution of properties exhibited by commercial cements
on the market and in clinical use. There was a wide range of
tensile strength values: Palacos R, Simplex P, and Sul-fix-60 gave the highest values of strength of approximately
50 MPa, CMW 3 gave a value of 44.7 MPa, while CMW1,
Boneloc, Osteobond, and Enduranceproduced strengths
of approximately 40 MPa. The value for Zimmer dough
type was the lowest at 31.7 MPa. The Palacos R, Simplex
P, and Sulfix-60 cements were significantly higher in
strength compared to the other cements tested with the ex-
ception of CMW3. There was no statistical difference be-
tween the values obtained for CMW 1, Boneloc, Osteo-
bond, and Endurance. The Young modulus results ranged
from 2.26 GPa for Boneloc to 3.53 GPa for CMW1.
Values for the strain to failure varied from 1.36% for
CMW3 to 2.48% for Boneloc.
The differences among fatigue results for the different
cements were much larger than those found with the static
tensile results. The highest Weibull median fatigue cycles to
failure obtained for Simplex P and Palacos R were con-
siderably higher than found for Zimmer dough type and
Boneloc. What is also important is the distribution of fatigue
lives. A narrow spread of fatigue lives is better than a wider
scatter of data, because it indicates greater predictability in
vivo. There is some correlation between the static and fatigue
Figure 1. Dimensions of tensile and fatigue test specimens.
TABLE II. Static Tensile Properties of Commercial Bone Cements
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TABLEIII.StatisticalDifferences
BetweenUltimateTensileStrengthsof
CommercialCementsObtainedUsingt
heStudentt-test
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strengths, but the ranking of static strength does not exactly
follow that of the fatigue lives.
Correlation of Results to Clinical Data
The method of fatigue testing employed in this investigation
is not designed to be a replica of the exact conditions expe-
rienced by a bone cement mantle surrounding a hip replace-
ment. The test does, however, assess the tensile fatigue prop-
erties of the various cements tested and, since bone cement is
weaker in tension compared to compression, it is more likely
to fail due to the tensile component of the cyclic loads when
subjected to them in vivo. Therefore, some correlation be-
tween the fatigue results and clinical data is to be expected.
The majority of investigations studying the loosening of hip
replacements do not compare the different cements tested. An
exception to this is the Swedish Hip Register.5
In the 1998review of 148,359 primary hip replacements, the authors
reported, Lowest risks are associated with Pallacos Genta-
mycin, plain Pallacos and Simplex. CMW has slightly
worse result with the highest risks associated with Sulfix.
This order of success is the same as that obtained from the
fatigue test carried out in this investigation, which is encour-
aging. It should also be noted that the cement with the
significantly lowest fatigue properties is Boneloc, a material
that was withdrawn from the market due to its high incidence
of loosening.
Factors Affecting Strength
The reasons for the differences obtained in mechanical prop-
erties can be attributed to variations in both composition of
polymer and monomer, particle size, morphology, and mo-
lecular weight of powder, strength of polymer bead-matrix
interface, and powder-to-liquid ratios. Cracks grow within
bone cement intergranularly (i.e., in the newly formed poly-
meric phase), transgranularly (i.e., in the preformed poly-
meric bead), and along the bead-matrix interface.12,13 There-
fore, both the powder and liquid components are important.
Another important influence upon the mechanical properties
is the method of sterilization. The sterilization technique used
for the majority of the cements is gamma irradiation, with the
exception of Palacos R, which is ethylene oxide sterilized,
and Sulfix-60 and Duracem 3, which are sterilized via
formaldehyde tablets. It has been shown in previous workthat gamma irradiation causes a large decrease in the strength
of a bone cement due to loss in molecular weight,14,15
whereas the use of ethylene oxide and formaldehyde does not
affect mechanical strength. Because there are many factors
influencing cement strength, it is not easy to interpret the
varying strengths of the cements. The main constituent of
Simplex P is a P(MMA/styrene) copolymer and this com-
position results in one of the highest values of both tensile
and fatigue strength. Osteobond copolymer cement is also
composed of a P(MMA/styrene) copolymer, but, although
TABLE IV. Fatigue Results of Commercial Bone Cements
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TABLEV.StatisticalDifferences
betweenFatiguePropertiesofCommercialCementsObtainedUsingtheMann
WhitneyU-Test
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this also has a relatively high fatigue resistance, it is not
significantly different from Duracem 3, CMW 3, and Sul-
fix-60, which are chiefly composed of PMMA. One major
factor is the ratio of PMMA to styrene in the copolymer, and
this information was not supplied. Palacos R also has out-
standing strength, with a relatively narrow distribution of
fatigue lives. The high strength may be attributed both to the
influence of the P(MMA/MA) copolymer and to the method
of sterilisation, i.e., via ethylene oxide, which does not have
a detrimental effect upon mechanical properties. The distri-
bution of fatigue life is influenced most by the presence of
flaws in the material and the resistance to propagation of
these flaws. Palacos R possessed a similar porosity on the
fracture surface to SimplexP, as assessed visually, suggest-
ing that its better distribution of fatigue life was due to it
being a tougher cement compared to most of the other ce-
Figure 2. Weibull distributions of fatigue cycles to failure after being cycled 0.322 MPa for (a) all
cements tested, (b) common commercial cements, and (c) newer commercial cements: () Simplex;
() Duracem; () CMW1; () Boneloc; () Palacos; () CMW3; () Endurance; ()Os-
teobond; () Sulfix-60; () Zimmer dough.
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ments tested.16 It is also important to note that, with the
exception of Boneloc, Palacosdisplayed the highest strain
to failure.
Another factor to consider is the value obtained for cement
modulus. Crowninshield et al.17 showed, using a 3-dimen-
sional finite element analysis model, that a lower cement
modulus results in lower stresses experienced by the cement
in vivo. However, the value of modulus does have to be
viewed in the context of both tensile and fatigue strengths. In
general, the cements with the highest values for modulus
possessed the higher values of tensile and fatigue strengths.
Comparison to Literature Studies
Relatively few investigations are reported in the literature
comparing a wide range of mechanical properties of com-
mercial bone cements using similar test protocols. A study by
Kusy, 1978,18 compared the tensile strengths of CMW,
Palacos, with and without gentamicin, Sulfix-6 and Sim-
plex P after 1 month and after conditioning for 10 month in
distilled water. The highest strength after 1 month was found
for the Sulfix-6 and Palacos, the lowest for the Simplex P.
After aging in distilled water, all the values of strength were
reduced, with the highest values for the CMWand Palacos
with gentamicin. There have been several investigations into
the mechanical properties of Simplex P compared to Zim-
mer dough and LVC, the most commonly used cements inthe USA. Weber and Bargar, 1983,19 reported no statistical
differences between the tensile strengths of these three ce-
ments after 14 days cure, but the Zimmercements displayed
lower flexural strengths. When tested in flexure, Simplex P
gave the highest result. In agreement with this result, was a
report by Davies et al., 1987,20 who reported no differences in
Figure 2. (continued)
Figure 3. Fatigue fracture surface of Palacos R bone cement. Figure 4. Fatigue fracture surface of Simplex P bone cement.
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strength when the cements were tested in tension. However,
after testing in tension-compression fatigue, under stress con-
trol, Simplex P possessed superior fatigue properties. Data
published by Gates et al., 1984,21 and Krause et al., 1988,22
showed that Zimmercements had inferior fatigue propertiesas compared to Simplex. In a study comparing the fatigue
characteristics of Palacos R and Simplex P by Davies et
al., 1989,23 no significant difference was obtained. A more
recent study by KindtLarsen et al., 1995,16 investigated a
range of commercial cements including CMW-1, Palacos
R, Simplex P, Zimmer dough and LVC, and Boneloc.
When mixed in an open bowl, Palacos R displayed the
highest value of tensile strength, followed by Simplex P;
Zimmerdough had the lowest strength. In flexure, Simplex
P had the highest value followed by Palacos R, again
Zimmer dough gave the lowest result. The fracture tough-
ness data showed Palacos R with highest value, the other
cements giving similar values, with the exception of Bone-
loc, which gave a value almost 50% lower. Fatigue proper-
ties of Simplex P in comparison to Boneloc were also
reported. When tested in strain control, Boneloc displayed
the highest cycles to failure, whereas in stress control, Sim-
plex P was superior. In view of the history of Boneloc in
clinical use in the body,24 fatigue testing under stress control
appears to be a better indicator of a prediction of clinical
success.
In summary, Simplex P and Palacos R generally pos-
sessed the highest strengths for both static and fatigue prop-erties, and Zimmer dough displayed the lowest values, in
agreement with results reported in this article for a wider
range of cements. The review of previous studies highlights
the problems of comparing data from different investigations,
which use varying methods and only a limited number of
bone cement formulations.
Scanning Electron Microscopy
The fatigue fracture surfaces examined via SEM revealed, in
general, relatively little plastic deformation occurred upon
fracture resulting in flat fracture surfaces. There was also, in
the majority of cases, evidence of the barium sulphate or
zirconium dioxide particles, added as a radiopaque filler,
having been pulled out of the surface upon failure. Failure
was often initiated from an internal pore.
Figure 3 shows a micrograph of a Palacos R cement
fracture surface, revealing zirconia particle pullout and rela-
tively smooth regions of bead fracture. The zirconia particles
Figure 5. Fatigue fracture surface of CMW 1 bone cement.
Figure 6. Fatigue fracture surface of CMW 1 bone cement at a
higher magnification.
Figure 7. Fatigue fracture surface of Osteobond co-polymer bone
cement.
Figure 8. Fatigue fracture surface of Sulfix-60 bone cement.
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tended to agglomerate and were much larger in size compared
to the barium sulphate particles used in other cements. In the
region around the pore that initiated failure, the surface was
rougher with more zirconia pull-out, indicating slow fracture.
Further from the pore, towards the edge of the sample the
surface was smoother, indicating fast fracture. Figure 4 is a
typical micrograph obtained for SimplexP cement, in which
it was possible to observe fracture around preformed beads
surrounded by the barium sulphate. This observation indi-cated that fracture had occurred through both the preformed
bead and newly formed inter-bead matrix, as described by
previous researchers.12,13 Barium sulphate did not show ev-
idence of agglomeration, unlike the zirconia in the Palacos
R. The cements CMW 1, CMW 3, and Endurance ap-
peared similar when viewed via the SEM. Figure 5 is a
typical example of the fracture surfaces, which were similar
to Figure 4. The CMW 3 cement surface was slightly
rougher in appearance, indicating the fracture was less brittle
than the CMW 1 and Endurance cements. Figure 6 shows
a higher magnification micrograph of the barium sulphate
particles. The particles appear to be situated in pores within
the PMMA matrix, with no evidence of any bond between the
two opposing surfaces. Zimmer dough cement fracture sur-
faces were similar to both the CMWand SimplexP. There
was, however, evidence of considerable porosity on the frac-
ture surfaces. Osteobond cement contained less pores com-
pared to the Zimmer cement and less barium sulphate pull-
out. Figure 7 shows a typical fracture surface revealing a
rougher surface compared to Figure 4. Sulfix-60 cement
fracture surfaces were different in appearance from the other
cements; there was little ceramic particle pull-out and the
surface gave evidence of less brittle failure. This observation
was supported by the strain-to-failure results shown in Table
II. The radiopacifying agent was zirconia, and particles areshown in Figure 8. Duracem 3 cement gave similar results to
Sulfix-60, not surprising since the compositions are very
similar. Boneloc cement fracture surfaces were different
from the majority of other cement fracture surfaces. The
surfaces showed evidence of a more ductile failure and there
were regions where there appeared to be separate layers of
material, as shown in Figure 9. There was very little evidence
of particle pull-out, but circular holes were observed in some
areas, from which, it was assumed, zirconia particles had
been pulled out. Some spherical particles remained in these
holes, as shown in Figure 10.
CONCLUSIONS
This study revealed that tensile and, in particular, fatigue tests
highlighted large differences in the strengths of the commer-
cial bone cements investigated. The cements that perform
best clinically gave the highest results in this study. In view
of this result, it appears important to test all experimental
bone cements in prescribed cyclic testing regimes in order to
evaluate their fatigue performance prior to use in surgery.This initial study used only one fatigue test condition for
sterilized cements prepared by hand mixing. However, the
value of the present study is in demonstrating independently
the wide range of fatigue performance in commercial bone
cements. To evaluate the cements fully, a comprehensive
study is in progress to assess cements prepared after vacuum
mixing as well as by hand mixing, with testing at 37C in
saline and for a wider range of stresses.
The IRC gratefully acknowledges the support of the EPSRC forits core grant. The authors also thank Dr. E. Dingeldein and Dr. H.Wahlig, Coripharm GmbH, Germany for their assistance in thesupply of some of the bone cements tested.
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