Biomaterials 24 (2003) 4071–4080

ARTICLE IN PRESS

*Correspondin

E-mail addres

0142-9612/03/$ -

doi:10.1016/S014

Elimination of barium sulphate from acrylic bone cements.Use of two iodine-containing monomers

A. Artolaa, M. Gurruchagaa, B. V!azquezb, J. San Rom!anb, I. Gonia,*aDpto. Ciencia y Tecnolog!ıa de Pol!ımeros, Facultad de Qu!ımica, POLYMAT (Instituto de materiales polim!ericos), Universidad del Pa!ıs Vasco,

Apdo 1072, 20080-San Sebasti !an, Spainb Instituto de Ciencia y Tecnolog!ıa de Pol!ımeros (CSIC), c/Juan de la Cierva, 3, 28006-Madrid, Spain

Received 26 September 2002; accepted 8 April 2003

Abstract

The aim of this work is to present a new approach to joint arthroplasty failure related to bone cement mantle. As barium sulphate

is considered one of the main causes of mechanical weakness in the cement, we substituted this inorganic radiopacifier of the solid

component for radiopaque monomers in the liquid component. We obtained two different cements, one containing 5 vol%2-[20; 30; 50-triiodobenzoyl] ethyl methacrylate (TIBMA) and the other containing 3,5-diiodine salicylic methacrylate (DISMA). Inboth cases, the mechanical properties of these new cements were better than those of the barium sulphate-containing cement. The

radiopacity obtained was comparable to that of the aforementioned cement and all the samples showed good biocompatibility.

r 2003 Elsevier Science Ltd. All rights reserved.

Keywords: Bone cement; Barium sulphate; Radiopaque monomer; Iodine-containing monomer; Mechanical properties; Biocompatibility

1. Introduction

PMMA-based biomaterials are usually made radio-paque by adding an inorganic contrast agent, such asbarium sulphate or zirconium dioxide. These agents donot mix well with the polymeric matrix for thermo-dynamic reasons, nevertheless, high concentrations (5–10% by weight) are routinely added to practically allbone cements formulations [1]. We know that theincompatibility between the polymeric and inorganicphases may affect the mechanical properties [2]. Thisdeficiency in polymeric materials has given rise toextensive literature describing various techniques usedto obtain some degree of X-ray opacity.Generally speaking, in these techniques opacity is

achieved by adding insoluble heavy metal salts, orga-nometallic compounds or organic opacifiers, but they donot constitute a viable alternative to barium sulphate.The use of organic opacifiers which are compatible withthe polymeric matrix is a step forward in this research[3]. The authors attribute the enhancement of themechanical properties to a much lower porosity of the

g author.

s: popgoeci@sq.ehu.es (I. Goni).

see front matter r 2003 Elsevier Science Ltd. All rights res

2-9612(03)00298-9

cement. Several authors [2,4] have suggested thatanother promising alternative for achieving radiopacityin biomaterials would be to modify polymers chemi-cally, introducing bromine or iodine to methacrylatemonomers. The polymerisation or copolymerisation ofsuch monomers would produce X-ray opaque materialswithout the disadvantages of two-phase systems. Var-ious monomers have been synthesised and copoly-merised giving rise to radiopaque materials to betested for application in the medical field. Benzina et al.[4] studied a new biomaterial obtained from thecopolymerisation of two methacrylates, methyl metha-crylate (MMA) and hydroxyethyl methacrylate, with athree iodine monomer: 2-[20; 30; 50-triiodobenzoyl] ethylmethacrylate (TIBMA). The polymer containing 7%TIBMA showed marked radiopacity and hemocompat-ibility. Over the last few years, several papers have beenpublished on the use of this kind of monomer in bonecement formulations. All of them were related to the useof acrylic monomers containing iodine to facilitate theintegration of the inorganic agents in the cement matrix.We can see that the addition of 5% of a methacrylateformulated with diiodo-hydroxyquinoline (IHQM) [5,6]or higher percentages of a different monomer alsocontaining an aromatic iodised structure [7,8] provided a

erved.

ARTICLE IN PRESSA. Artola et al. / Biomaterials 24 (2003) 4071–40804072

significant increase in the tensile properties. In thesepapers IHQM has been extensively studied showing thatalthough the tensile strength and the fracture toughnessincreased, the fatigue crack propagation remained aslow as it was for the radiolucent cement. Thus, wethought it would be worthwhile pursuing this particularroute of investigation.The monomer proposed by Benzina et al. [4], namely

TIBMA (Fig. 1a), has been selected in this work toobtain radiopaque acrylic bone cements. A secondmonomer, synthesised in our laboratory, was also used,i.e. the 3,5-diiodine salicylic methacrylate (D1SMA)(Fig. 1b). The methacrylate structure is very interestingbecause polymerisation take place with relative ease.Moreover, the use of this kind of monomer forcopolymerisation with MMA introduces fewer changesthan the use of other kinds of vinylic monomers. Inaddition, aromatic derivatives enable the presence ofmore than one atom of iodine in the same molecule,resulting in high radiopacity. All of these factors led usto synthesise TIBMA and DISMA. In the latter case,the possibility of taking advantage of the anti-inflam-matory nature of salicylic acid was also taken intoaccount. Several papers noted that copolymers contain-ing salicylic acid derivatives are able to release the drugin mild conditions by hydrolytical processes [9]. More-over, these copolymers demonstrated sufficient hydro-lytical stability for long term applications [10].In this research, the new cements obtained with both

monomers were compared with a conventional formula-tion, using 10% barium sulphate and a radiolucentformulation containing only polymeric ingredients. In aprevious paper [8], we already published results obtainedwhen making cement formulations with a differentiodine-containing monomer: the 4-iodophenol metha-crylate (IPMA). Cements formulated by substituting 0–20 vol% MMA for IPMA were studied, showing thatthis approach was interesting for preparing other kindsof radiopaque cement formulations. As we explain

OCH3

O

O

O

O

CH3

O

COOH

(a)

(b)

I I

I

I

I

Fig. 1. TIBMA and DISMA monomers structure.

below, in this paper only 5 vol% of the respectivemonomer was added to the formulation.

2. Materials and methods

2.1. Materials

2.1.1. Monomers

Iodine monomers were synthesised in our laboratoriesfollowing the method described by other authors:

TIBMA: synthesis was carried out following therecipe published by Benzina et al. [4]. First, wesynthesised 2,3,5-triiodobenzoyl chloride from thio-nylchloride (Aldrich) and 2,3,5-triiodobenzoic acid(Aldrich). THF (Panreac) and n-hexane (Panreac)solvents used in the process were freshly distilled fromsodium. All of the others products were reagent gradeand were used as received.Next, the acid chloride, the hydroxyethyl methacrylate

(Merck), previously distilled under vacuum, and thetriethylamine (Merck) were allowed to react in drydichloromethane (Panreac). After recrystallisation, weobtained TIBMA as off-white crystals with a melting pointof 92-93�C that were identified by 1H-NMR and 13C-NMR spectroscopies. The reaction yield was around 95%.

DISMA: The synthesis reaction was carried outfollowing the acid chloride method [11]. 25:7 mmol ofmethacryloil chloride (Fluka) was added dropwise to astirred solution of 12:8 mmol of 3,5-diiodosalycilic acidand 13:0 mmol of dry triethylamine in 30 ml of ethylicether. Then the mixture was allowed to react for 48hours and filtered and concentrated. Heptane was addedto act as a precipitant. A white powder with a meltingpoint of 97298�C was obtained after a new filtration,namely 3,5-diiodosalicylic acid methacrylate, which wasidentified by 1H-NMR and 13C-NMR spectroscopies.The yield obtained was 25%, approximately. This lowyield is attributed to the formation of a solid by-productwhich precipitates together with the DISMA in the firststep. The purification of the DISMA by a solution/precipitation method entails a big loss of DISMA.

2.1.2. Cements

The experimental bone cements were formulated frompoly(methyl methacrylate) (PMMA) beads of 69:4 mmaverage diameter (Colacryl DP 300) supplied by Bonar.The PMMA beads were mixed with 1.25% (w/w)benzoyl peroxide (BPO) (Merck) to form the solidcomponent of the cement. 10% (w/w) barium sulphate(Merck) was incorporated to one of the formulations.The liquid component consisted of 1% (v/v) dimethyl

p-toluidine (DMPT) activator (Merck) and MMAmonomer (Merck). 5% (v/v) of the monomer wassubstituted by the equivalent amount of TIBMA orDISMA. In both cases, the addition of greater amounts

ARTICLE IN PRESSA. Artola et al. / Biomaterials 24 (2003) 4071–4080 4073

of the radiopaque monomer led to problems in thecopolymerisation. These problems will be studied in afuture work, preparing the copolymers at low conver-sion. The samples were obtained using conventionalquantities and a 2/1 solid/liquid ratio.Thus, in order to compare the effect of both

monomers, we prepared four different cement series:

1.

Radiolucent cement: without any type of radio-pacifier i.e. only PMMA beads and BPO as the solidcomponent, and MMA and DMPT as the liquidcomponent.

2.

Conventional cement: incorporating 10% (w/w) ofbarium sulphate to the solid component.

3.

TIBMA cement: substituting 5% (v/v) MMA by5% (v/v) TIBMA in the liquid phase.

4.

DISMA cement: substituting 5% (v/v) MMA by5% (v/v) DISMA in the liquid phase.

2.2. Methods

2.2.1. Setting parameters

The kinetics were followed at 23�C; according to ISOStandard, and at 37�C to record data at bodytemperature. The exothermic polymerisation tempera-ture was recorded using the procedure explained inprevious papers [12], analogous to that of ISO-5833Standard [13]. Time and temperature were measuredfrom the onset of mixing the powder with the liquid.Three determinations were performed for each of thecements. The differences between formulations werestatistically established.

2.2.2. Residual monomer

The residual monomer content was determined by1H-NMR spectroscopy. The samples were allowed tocure for 1 h at 37�C before being dissolved in deuteratedchloroform. The spectra for three samples of each typewere recorded on a Varian VXR-300 spectrometeroperating at 75:5 MHz: Chloroform signal at 7.27 dwas used as an internal chemical-shift reference.The chemical shifts attributed to the protons of the

vinylic methylenes of the monomers and the shiftattributed to the a-methyl group of monomers as wellas that of polymeric structural units were both used tocalculate the residual monomer. With respect to theprotons of the vinylic methylenes, the following assign-ments were used: shifts at 5.58 and 6.10 d assigned toMMA, those of the vinylic methylenes pertaining toTIBMA at 5.45 and 6.0 d and, for the vinylic protons ofthe DISMA, the shifts that appear at 5.84 and 6.44 d:

2.2.3. Dynamic and static mechanical properties

Mechanical properties have been determined for dryand wet samples. For the testing of wet samples, each

specimen was maintained in saline solution at 37�C untilequilibrium uptake, gravimetrically determined, wasattained (after a week approximately). The staticcompression tests were conducted using specimenconfiguration, specimen dimensions and procedures asdetailed in the ISO 5833 (1992) Standard [13]. Thus, thespecimens were in the form of a short cylinder(6 mm + and 12 mm high). The tests were performedat a cross-head displacement speed of 22 mm=min on aMTS-Bionix 858 testing machine with a cell load of100 KN:Tensile tests of all specimens were also carried out on

a MTS-Bionix 858 machine, with a cell load of 2:5 KNat a cross-head speed of 1 mm=min: An extensometerwas used to measure strain. Specimens were prepared byplacing the cement dough in PTFE moulds and by,subsequently, putting them in an oven for 1 h at 37�Cunder pressure. The tensile samples were preparedaccording to the UNE 5300 386 standard specifications(sample A).For the static mechanical analysis, a total of six

cement samples of each formulation were tested. Themean and standard deviation values were calculatedusing the 1-way ANOVA statistical technique. Thistechnique assesses the variance between groups, which isthen compared against the error variance to test whetherit is significantly different. The error protection methodused in this investigation was the Fisher PLSD methodand the confidence interval used was 95%.Loss and storage modules depend on the chains’

structural characteristics and the morphology of multi-phase systems, and can be measured by means ofdynamic mechanical thermal analysis (DMTA). DMTAwas carried out in bending mode from 20 to 200�C usinga DMTA MK III instrument. The specimen size was:40 mm� 10 mm� 1:5 mm; the frequency was 1 Hzwith 0:064 mm displacement and the heating rate was3�C=min: Glass transition temperature of the cementswas read off as the temperature at which the lossmodulus or loss factor passed through a maximum. Thistechnique provides values of Tgs that can be visiblyhigher than those measured by Differencial ScanningCalorimetry (DSC).Representative tensile fracture samples were selected

for examination by a scanning electron microscope(SEM). The study was carried out using a JEOL 6300TM

microscope. All samples were gold coated to beexamined using an accelerating voltage of 10 kV at awide range of magnifications. The porosity of thesamples was not a problem in the selection of samples,because of the number and the extension (o100microns) of the pores was very small.

2.2.4. Radiopacity and biocompatibility

Obviously, in this research it was necessary toascertain the radiopacity obtained with each monomer.

ARTICLE IN PRESS

Fig. 2. 1H-NMR spectrum of TIBMA monomer. TMS is used as

internal reference.

A. Artola et al. / Biomaterials 24 (2003) 4071–40804074

A radiographic study was carried out with a standardclinical General Electric X-ray instrument (set at 55 kVand 2.5 MAS). The relative X-ray opacity wasdetermined visually by comparison of the new cementswith the sample containing barium sulphate. 1 mm thickspecimens were used.In vivo studies of biocompatibility of the acrylic bone

cements were also carried out. The animal experimentswere performed with female Wistar rats weighing300710 g: Rods of cured cement (3 mm diameter�15 mm in length) were cannulated in the dorsal muscleof the animals, in order to produce minimum injury. Thewound was sewn with a stitch of suture and disinfectedwith Betadines: Again, the four types of cements wereanalysed: radiolucent PMMA (Colacryl), radiopaquePMMA containing barium sulphate, and two radio-paque PMMA cements containing an acrylic radio-pacifier: 5% DISMA and 5% TIBMA. Four animalsimplanted with cement of each composition wereoperated on and 10 weeks later they were sacrificed.The samples for histological preparation were fixed

first in 10% neutral buffered formalin (phosphatebuffered, pH ¼ 7:6) and embedded in paraffin wax.Thin ground sections (6 mm thickness) were preparedwith a manual microtome (Minot-Leniz) and thesections were stained by the haematoxylin and eosintechnique for histological examination. The sampleswere examined under a Nikon Microphot-FXA opticalmicroscope.

Fig. 3. 1H-NMR spectrum of DISMA monomer. TMS is used as

internal reference.

20

30

40

50

60

0 500 1000 1500 2000 2500 3000 3500

COLACRYL5% TIBMA10% BaSO45% DISMA

Tem

pera

ture

(°C

)

Time (s)

Fig. 4. Kinetics of the setting at 23�C of 5% TIBMA, 5% DISMA

and 10% barium sulphate containing cements and radiolucent one.

3. Results and discussion

The reaction rates for copolymerisation reactionsdepend on the structure of the monomers (Fig. 1). Inthis case, both monomers are basically aromaticmethacrylates, the first one, TIBMA, contains twomethylene groups, the second one, DISMA, contains acarboxyl group. Moreover, TIBMA contains threeiodine atoms and DISMA two iodine atoms. Theobtained monomers were identified by 1H-NMR and13C-NMR spectroscopies. Figs. 2 and 3 display their1H-NMR spectra.Figs. 4 and 5 show the kinetics of polymerisation

observed at 23�C and 37�C; respectively. We can see theeffect of the addition of the radiopaque monomerscompared with formulations containing barium sul-phate and PMMA radiolucent formulations. Althoughpractically no changes can be observed in the maximumtemperature attained, we can clearly appreciate that theeffect of the monomers in the setting time is morenoticeable and it is exactly the opposite to that of thebarium sulphate. With respect to Fig. 4, we can say thatthe influence of the new monomers is greater than thatof the addition of the barium sulphate. This is a logicalresult since the inorganic agent does not take part in the

polymerisation process unlike iodine monomers. Thehighest maximum temperature attained is attributable tothe formulation containing 5% TIBMA, and the lowestto the formulation with 5% DISMA, the difference ofthe latter with the conventional formulation is statisti-cally significant. The rate of copolymerization of these

ARTICLE IN PRESS

20

30

40

50

60

0 500 1000 1500 2000 2500 3000 3500

COLACRYL10% BaSO45% TIBMA5% DISMA

Tem

pera

ture

(°C

)

Time (s)

Fig. 5. Kinetics of the setting at 37�C of 5% TIBMA, 5% DISMA,

10% barium sulphate containing cements and radiolucent one.

able 1

etting parameters obtained for cements polymerised at 23�C and at

ody temperature. Results are presented as mean (standard deviation)

omposition 23�C 37�C

tsetting ðminÞ Tmax ð�CÞ tsetting ðminÞ Tmax ð�CÞ

olacryl 34.38 (0.97) 41.7 (0.5) 12.62 (0.07) 56.2 (0.5)

% TIBMA 29.25 (0.93) 42.6 (0.2) 11.62 (0.65) 58.7 (1.3)

% DISMA 32.92 (0.42) 39.8 (0.4) 11.79 (0.88) 58.8 (1.7)

0% BaSO4 37.88 (0.38) 41.6 (0.2) 12.83 (1.17) 59.8 (1.5)

A. Artola et al. / Biomaterials 24 (2003) 4071–4080 4075

formulations was higher than the rate of those contain-ing only MMA. As we have mentioned before, thehigher reactivity and the copolymerization kinetics willbe studied in more depth in a future paper. In Fig. 5, wecan observe that the increase in the polymerisationtemperature affects the four formulations in a similarway and that all four present the same evolution. In thecase of all four, the maximum temperature shown ishigher and the polymerisation times are lower than at23�C:By applying the ISO formula to calculate the setting

time, we obtained the results in Table 1. A majordifference can be observed between the formulationscontaining barium sulphate and those containing 5%TIBMA. While, on this table, neither the new formula-tions nor the standard formulation with the inorganicradiopaque agent comply with the ISO requirements, wemust point out that this is due to the use of large

PMMA beads [12] and a different mould than thatdescribed in the standard. Despite this, taking intoaccount the value of setting time of the standardformulation as a reference, the decrease of 8 min: forthe 5% TIBMA-containing formulation and 4 min:for the 5% DISMA-containing formulation one canbe considered quite significant, moreover these differ-ences are statistically significant ðpo0:0001Þ: The size/size distribution of the PMMA beads [12] can alsoexplain the low maximum temperature attained in allcases.Another very important factor when evaluating bone

cements is the free monomer remaining in the cement,which can eventually be released into the organism.Figs. 6 and 7 show the 1H-NMR spectra of bothformulations. As commented before, the use of theappropriate signals allowed a value of 2.1% of mono-mer in the formulation containing TIBMA and 2.6% ofmonomer in the cement containing DISMA, calculatedin line with conventional formulations. As we can see inthe spectra the amount of iodine-containing monomer isclearly smaller than that of the MMA. Various studies

T

S

b

C

C

5

5

1

have been carried out to establish a relationship betweenthe structure and the toxicity of monomers used inbiomedical applications. Neither TIBMA nor DISMAhas been tested yet, but we can analyse them from somegeneralisations made by other authors. Toxicity hasbeen related to water solubility [14,15] and lipophilicity[16] and hemolitic potency to the hydrophobic nature ofthe monomers [17]. Moreover, other studies [18]conclude that acrylates are more toxic than methacry-lates with corresponding substituents and that hydroxylgroups enhanced the citotoxicity of acrylates andmethacrylates. The effect of iodine atoms in toxicityhas also been discussed. Our monomers are methacry-lates which are non-soluble in water and do not havehydroxyl groups, so we do not expect acute toxicityrelated to these characteristics to appear. However,more studies about the effects of their hydrophobicnature are needed. Finally, no toxicity seems to beinduced by iodine at working concentrations [19].Dynamic mechanical measurements are very useful in

order to characterise the linear viscoelastic behaviour ofpolymers. Glass transition temperature ðTgÞ is a para-meter strongly associated to the behaviour of polymers.From among other characteristics, Tg depends on themolecular weight, water content and molecular structureof the monomers used. Acrylic polymers absorb water ina non-negligible degree depending on the type andnumber of monomeric units. Since bone cements arealways in a humid environment at 37�C; measurementsof Tg in a dry state and after equilibrium uptake isattained are of great interest. In this work, dynamicmechanical thermal analysis (DMTA) was conducted todetermine the effect of the addition of the radiopaquemonomers. As we discuss below, our specimens weresubjected to DMTA tests in a dry state as well as afterreaching the equilibrium water uptake in distilled water.Table 2 shows the values of water (WA) and salinesolution absorption (SA), and the Tg of the fourformulations compared in this work, calculated fromDMTA results.Formulations containing the radiopaque monomers

absorb more water than the conventional formulation.

ARTICLE IN PRESS

Fig. 6. 1H-NMR spectrum of TIBMA containing cement. Chloroform is used as internal reference.

Fig. 7. 1H-NMR spectrum of DISMA containing cement. Chloroform is used as internal reference.

Table 2

Percentage of water (% WA) and fisiologic saline solution (% SA)

absorbed at 37�C at the equilibrium uptake. Tgs for the dry and wet

states of the new cements, radiolucent and conventional one. Results

are presented as mean (standard deviation)

% WA % SA Tg dry Tg wet

Colacryl 1.86 1.80 127.23 115.32

(0.01) (0.01) (0.05) (0.28)

BaSO4(10%) 1.63 1.55 128.98 118.28

(0.01) (0.09) (0.42) (0.73)

5% TIBMA 1.76 1.69 128.80 117.06

(0.04) (0.09) (0.66) (0.85)

5% DISMA 1.82 1.94 129.64 116.47

(0.08) (0.02) (0.50) (0.22)

A. Artola et al. / Biomaterials 24 (2003) 4071–40804076

It is more interesting to compare the absorptionbehaviour in water and in saline solution. While in thecase of 5% DISMA formulations the absorption ishigher, in the other cases a decrease is observed due tothe osmotic process produced in a saline medium. Thisdifference, which is statistically significant ðpo0:0018Þ;and the higher absorption of the DISMA-containingformulation are probably due to the presence of thecarboxyl group in the monomeric structure, whichproduces salts in contact with the saline solution. Onthe other hand, no appreciable differences in the Tg ofthe various formulations can be seen, only a smallincrease due to the low percentage of aromaticstructures is observed in the new formulations. How-ever, the plasticizing effect created by water uptake leadsinevitably to a Tg decrease in all cases [20].

ARTICLE IN PRESS

0

0.5

1

1.5

0 50 100 150 200

Colacryl5% TIBMA5% DISMA10% BaSO4

tan

δ

Temperature (°C)

0 50 100 150 2006

7

8

9

10

log

E' (

MP

a)

Temperature (°C)

(a)

(b)

Fig. 8. DMTA study of the four cements formulations in the dry state:

(a) loss modulus and (b) elastic modulus.

6

7

8

9

10

11

0 50 100 150 200

log

E' (

MP

a)

Temperature (°C)

0

0.5

1

1.5

0 50 100 150 200

Colacryl5% TIBMA5% DISMA10% BaSO4

tan

δ

Temperature (°C)(a)

(b)

Fig. 9. DMTA study of the four cements formulations in the wet state:

(a) loss modulus and (b) elastic modulus.

A. Artola et al. / Biomaterials 24 (2003) 4071–4080 4077

Concerning the DMTA study, in Fig. 8 we can observe achange in the loss peak associated with the glass transitionof the PMMA when copolymerised with DISMA andTIBMA. The small changes observed are to be expectedconsidering the small difference in the free volume that canbe introduced when using only 5% comonomer [21]. Thissame behaviour is observed in the wet state (Fig. 9). As wehave mentioned before, the plasticizing effect of waterdecreases the temperature of the glass transition and it alsobroadens the loss peak. The extent of this broadeningdepends on the nature of the interaction between thepolymer and the plasticizer. The broad damping peaksobserved can be attributed to the limited solubility of waterin this type of acrylic polymers. As the plasticizer becomes apoorer solvent, a broader peak should be observed. Thus,we cannot affirm that the plasticizing effect is greater in theDISMA-containing formulation than in the other acrylicspecimens. Although the Tgs shown are far above bodytemperature, the influence of the plasticizing effect in thestatic mechanical properties, such as the compressive andtensile properties, must be evaluated. Tables 3 and 4 displaythe results of these tests.With respect to the compressive test (Table 3), in the

dry state we can point out that the most noticeabledifferences between the conventional cement and the

new formulations are found in the maximum strengthvalues. The smax obtained for the formulation contain-ing the inorganic component is very similar to that ofthe radiolucent cement. However, radiopaque polymericformulations show a higher value, which is statisticallysignificant ðpo0:0001Þ: Elastic modulus is less affected.In the wet state, similar behaviour to the dry state and adecrease of both parameters are observed. We can alsosay that the values given by the new formulations arehigher than those given by the conventional cement, andthe differences found in the modulus are statisticallysignificant ðpo0:0002Þ for both formulations.Table 4 shows the data collected from the tensile tests

using dry and wet specimens. The values for themodulus and the maximum tensile strength of the newradiopaque cements are closer to those of the radi-olucent cement than to the conventional cement. Inaddition, in spite of the expected reinforcing effect of theinorganic particles in the polymeric matrix, tensilevalues of the new cements are higher than that of thebarium sulphate-containing cement. This enhancement,reflected in the three parameters shown in Table 4, isstatistically significant ðpo0:0001Þ for the DISMA-containing formulation. Also, this behaviour is analo-gous for the modulus and the maximum tensile strength

ARTICLE IN PRESS

Table 3

Mechanical parameters obtained from the compressive test. Results

are presented as mean (standard deviation)

Dry Wet

smax (MPa) E ðMPaÞ smax (MPa) E ðMPaÞ

Colacryl 120.58 2261 111.12 2055

(1.31) (87) (2.94) (115)

BaSO4 (10%) 120.46 2138 113.69 1877

(3.12) (132) (3.42) (32)

5% TIBMA 126.79 2454 118.09 2291

(1.42) (118) (3.84) (147)

5% DISMA 128.64 2248 114.29 2035

(1.37) (194) (2.25) (169)

Table 4

Mechanical parameters obtained from the tensile test. Results are

presented as mean (standard deviation)

Dry Wet

smax E e smax E e(MPa) (MPa) (%) (MPa) (MPa) (%)

Colacryl 45.64 2143 2.78 41.73 2070 2.85

(2.12) (36) (0.39) (2.31) (35) (0.25)

BaSO4 (10%) 34.29 2255 1.93 36.75 1983 2.33

(1.40) (86) (0.17) (4.10) (67) (0.30)

5% TIBMA 45.78 2330 2.23 43.10 2048 2.87

(0.96) (51) (0.23) (3.72) (60) (0.12)

5% DISMA 46.66 2255 2.46 43.32 2114 2.72

(2.30) (164) (0.21) (3.07) (35) (0.18)

Fig. 10. SEM fractographies from the tensile fracture surface of:

(a) conventional cement, (b) TIBMA-containing cement and

(c) DISMA-containing cement.

A. Artola et al. / Biomaterials 24 (2003) 4071–40804078

of the TIBMA-containing formulation. As in thecompressive test, a decrease of the smax and the elasticmodulus is observed when we pass from the dry to thewet state. Moreover, the plasticizing effect, obviously,produces an increase in the deformation.Representative fracture tensile samples were selected

for examination by scanning electron microscopy. SEMfractographies of the radiopaque cements look quitedifferent (Fig. 10). In Fig. 10a, we can clearly see theheterogeneity produced by the barium sulphate particleswhich carries out the formation of pores and which is acommonly reported feature [22]. PMMA beads appearcut and some crazing can be appreciated. Fig. 10b showsthe fracture of a specimen containing 5% TIBMA. Wecan see close interaction at the interface betweenPMMA beads and the copolymer matrix. The fracturesurface cuts the beads and some crazing is produced.In the case of the specimen containing 5% DISMA(Fig. 10c), most of the PMMA beads are well integratedin the copolymer matrix and appeared well cut. In thiscement again crazing is present.

Clearly, radiopacity and biocompatibility are keyfeatures of bone cements in this study. The findings forthe former are illustrated in Fig. 11. This figure showsthe photograph obtained by X-rays of both new cementstogether with a conventional one containing, as in theprevious tests on this work, 10% barium sulphate. Thethree samples offer good radiopacity. The radiopacitybecomes more homogeneous when caused by thecopolymer matrix than when we use the inorganicagent, although the cement with inorganic agent showsslightly higher radiopacity.

ARTICLE IN PRESS

Fig. 11. X-ray photographies of 5% TIBMA, 5% DISMA, 10%

barium sulphate containing cements and radiolucent one.

Fig. 12. Histological samples of tissue surrounding cylindrical speci-

mens of: (a) 5% TIBMA and (b) 5% DISMA containing cements (c)

10% barium sulphate containing cement, where A is the fibrotic

capsule.

A. Artola et al. / Biomaterials 24 (2003) 4071–4080 4079

For this study, biocompatibility of our acrylicformulations was evaluated by implanting rods ofcement cured ‘‘in vitro’’ in the dorsal muscle of ratsfor a period of 10 weeks. Ten weeks is long enough toreach the steady state [23]. The response observed in theconnective tissue, when the experimental cements werein place, was comparable to that obtained after theimplantation of conventional bone cements (Fig. 12).Necrotic areas were not observed in either of theformulations studied. The tissue surrounding the mate-rial was mainly formed by connective tissue, rich incollagen fibres, which were aligned in the longitudinaldirection of the implant. We can point out that cementscontaining barium sulphate show a higher inflammatoryresponse than the new cements containing eitherDISMA or TIBMA; we can also observe a higherneoformation of blood vessels and a higher number ofinflammatory cells reinforcing the fibrotic capsule turn-ing it thicker. The fibrous membrane formed by thepresence of rods containing DISMA and TIBMA werethin and without new blood vessels. This kind ofresponse is more noticeable in the case of DISMA-containing cement. In general, it can be said that theresponse of the tissues to the implantation of the cementcontaining radiopaque monomers did not differ sig-

nificantly from that obtained with commerciallyavailable cements, and that their behaviour can beconsidered normal as an implant material. These resultsare in keeping with the findings of other authors onTIBMA terpolymer [24].

4. Conclusions

The addition of radiopaque monomers does not havea negative effect on the setting kinetics of the cements.

ARTICLE IN PRESSA. Artola et al. / Biomaterials 24 (2003) 4071–40804080

We can also say that both monomers exhibit goodradiopacity and biocompatibility.Alternatively, there are tensile mechanical properties

that clearly show the negative effect of the inorganicradiopaque agent. Therefore, in our case, we can saythat in both cases we obtained bone cements withenhanced mechanical properties, particularly with re-spect to the maximum compressive and tensile strength.Also, the elastic modulus in both tests showed a highervalue with the new monomer formulations. In spite ofthe fact that the deformation values are not included inthe ISO standard, they are a key feature when studyingcement life. In the tensile test, the decrease in deforma-tion normally produced by the addition of bariumsulphate was compensated for by the addition of bothmonomers.In the light of what has been discussed in this work,

we can say that we have reached the objectives proposedin our research: to achieve new radiopaque acrylic bonecements with better mechanical properties than conven-tional ones. In order to confirm these results furtherwork must be done. A study of fatigue life, fracturetoughness and crack propagation performance isnecessary and, in addition, the fact that an enhancementof the mechanical properties gives rise to enhancementof the ‘‘in vivo’’ performance of the cements must beproven.

Acknowledgements

The financial support has been granted by theComisi !on Interministerial de Ciencia y Tecnolog!ıa(CICYT) through the project MAT1999-1064-C02-01.The authors also wish to thank the University of theBasque Country for their support (9/UPV 00203.215-13540/2001). We wish to thank Dr. A. L !opez-Bravofrom the Hospital General from Avila for the histolo-gical study.

References

[1] Bulstra SK, Benzina A, Kuijer R, Koole LH. Novel screw caps for

use in orthopaedic and trauma surgery. Med Dev Technol

1999;10(3):38–9.

[2] Davy KWM, Anseau MR, Odlyha M, Foster GM. X-ray opaque

methacrylate polymers for biomedical applications. Polym Int

1997;43:143–54.

[3] Deb S, Abdulghani S, Behiri JC. Radiopacity in bone cements

using organobismuth compound. Biomaterials 2002;23:3387–93.

[4] Benzina A, Kruft M-AB, van der Veen FH, B.ar FHM, Breezer R,

Lindhout T, Koole LH. A versatile three-iodine molecular

building block leading to new radiopaque polymeric biomaterials.

J Biomed Mat Res 1996;32:459–66.

[5] V!azquez B, Ginebra MP, Gil FJ, Planell JA, L !opez-Bravo A,

San Rom!an J. Radiopaque acrylic cements prepared with a

new acrylic derivative of yodoquinoline. Biomaterials 1999;20:

2047–53.

[6] Ginebra MP, Albuixech L, Fern!andez-Barrag!an E, Aparicio C,

Gil FJ, San Rom!an J, V!azquez B, Planell JA. Mechanical

performance of acrylic bone cements containing different radio-

pacifying agents. Biomaterials 2002;23:1873–82.

[7] Artola A, Gurruchaga M, Goni I, Ginebra MP, Gil FJ, Planell

JA. Studies on two new radiopaque methacrylic bone cements,

2000 World Biomaterials Congress, Hawaii, USA, 2000.

[8] Artola A, Goni I, Gil J, Manero JM, Gurruchaga M. A

radiopaque polymeric matrix for acrylic bone cements. J Biomed

Mat Res (Appl ed) 2003;64B(1):44–55.

[9] Liso PA, Rebuelta M, San Rom!an J, Gallardo A, Villar M.

Antinociceptive and antipyretic properties of a new conjugated

ibuprofen-methacrylic polymeric controlled delivery system.

J Cont Rel 1995;33(3):429–36.

[10] Elvira C, Gallardo A, Lacroix N, Schacht E, San Rom!an J.

Incorporation of salicylic acid derivatives to hydrophilic copoly-

mer systems with biomedical applications. J Mater Sci Mater Med

2001;12:535–42.

[11] Truce WE, Bailey PS. The mechanism of alcoholysis of carboxylic

acid halides in the presence of triethylamine. J Org Chem

1969;34:1341.

[12] Pascual B, V!azquez B, Gurruchaga M, Goni I, Ginebra MP, Gil

FJ, Planell JA, Levenfeld B, San Rom!an J. New aspects of the

effect of size and size distribution on the setting parameters and

mechanical properties of acrylic bone cements. Biomaterials

1996;17:509–16.

[13] ISO. International standard 5833/2: Implants for surgery-acrylic

resin cements. Orthopaedic application, 1992.

[14] Lawrence WH, Bas GE, Purcell WP, Autian J. Use of

mathematical models in the study of structure-toxicity relation-

ships of dental compounds. I. Esters of acrylic and methacrylic

acids. J Dent Res 1972;51:526–35.

[15] Bass GE, Lawrence WH, Purcell WP, Autian J. Further

evaluation of a quantitative mathematical model for predicting

acute toxicity of acrylate and methacrylate esters. J Dent Res

1972;51:1632–8.

[16] Dillingham EO, Laurence WH, Autian J. Acrylate and metha-

crylate esters: relationship of hemolitic activity and in vivo

toxicity. J Biomed Mater Res 1983;17:945–57.

[17] Fujisawa S, Imai Y, Kojima K, Masuhara E. Studies on hemolitic

activity of bisphenol A diglycidil methacrylate (BIS-GMA).

J Dent Res 1978;57:98–102.

[18] Yoshii E. Cytotoxic effects of acrylates and methacrylates:

relationship of monomer structures and citotoxicity. J Biomed

Mater Res 1997;37:517–24.

[19] Sherer TT, Thrall KD, Bull RJ. Comparison of toxicity by iodine

and iodide in male and female rats. J Toxicol Environ Health

1991;32(1):89–101.

[20] Moore RJ, Flick JR. The influence of water concentration on the

mechanical and rheo-optical properties of poly(methyl methacry-

late). In: Rowland de JP, editor. Water in polymers. ACS

Symposium Series, USA, 1980. p. 555–69.

[21] Ward IM, Hadley DW. An introduction to the mechanical

properties of solid polymers. Chichester, UK: Wiley; 1995.

p. 173–93.

[22] Bhambri SK, Gilbertson LN. Micromechanism of fatigue crack

initiation and propagation in bone cements. J Biomed Mater Res

1995;29:233–7.

[23] Revell PA, Braden M, Freeman MAR. Review of the biological

response to a novel bone cement containing poly(ethyl metha-

crylate) and n-butyl methacrylate. Biomaterials 1998;19:1579–86.

[24] Kruft MAB, Van der Veen FH, Koole LH. In vivo tissue

compatibility of two new radio-opaque polymeric materials.

Biomaterials 1997;18:31–6.