caracterizarea a 10 cimenturi comerciale

8/12/2019 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.

605


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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

606 HARPER AND BONFIELD


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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

607TENSILE CHARACTERISTICS OF ACRYLIC BONE CEMENTS


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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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Figure 10. Fatigue fracture surface of Boneloc bone cement show-

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