Characterization of New Acrylic Bone Cements Prepared withOleic Acid Derivatives

Blanca Vazquez,1 Sanjukta Deb,2 William Bonfield,3 Julio San Roman1

1 Instituto de Ciencia y Tecnologıa de Polımeros, Juan de la Cierva, 3 28006 Madrid, Spain

2 GKT Dental Institute, King’s College, London Bridge, London SE1 9RT, United Kingdom

3 Department of Materials Science and Metallurgy, University of Cambridge, Pembroke Street, Cambridge CB2 3QZ,United Kingdom

Received 15 June 2001; revised 24 September 2001; accepted 26 September 2001Published online 00 Month 2001

Abstract: Acrylic bone-cement formulations were prepared with the use of a new tertiaryaromatic amine derived from oleic acid, and also by incorporating an acrylic monomerderived from the same acid with the aim of reducing the leaching of toxic residuals andimproving mechanical properties. 4-N,N dimethylaminobenzyl oleate (DMAO) was used as anactivator in the benzoyl-peroxide radical cold curing of polymethyl methacrylate. Cementsthat contained DMAO exhibited much lower polymerization exotherm values, ranging be-tween 55 and 62 °C, with a setting time around 16–17 min, depending on the amine/BPOmolar ratio of the formulation. On curing a commercial bone cement, Palacos� R with DMAO,a decrease of 20 °C in peak temperature and an increase in setting time of 7 min wereobtained, the curing parameters remaining well within limits permitted by the standards. Ina second stage, partial substitution of MMA by oleyloxyethyl methacrylate (OMA) in theacrylic formulations was performed, the polymerization being initiated with the DMAO/BPOredox system. These formulations exhibited longer setting times and lower peak temperatureswith respect to those based on PMMA. The glass transition temperature of the experimentalcements were lower than that of PMMA cement because of the presence of long aliphaticchains of both activator and monomer in the cement matrix. Number average molecularweights of the cured cements were in the range of 1.2�105. PMMA cements cured withDMAO/BPO revealed a significant (p<0.001) increase in the strain to failure and a significant(p<0.001) decrease in Young’s modulus in comparison to Palacos� R, whereas ultimate tensilestrength remained unchanged. When the monomer OMA was incorporated, low concentra-tions of OMA provided a significant increase in tensile strength and elastic modulus withoutimpairing the strain to failure. The results demonstrate that the experimental cements basedon DMAO and OMA have excellent promise for use as orthopaedic and/or dental groutingmaterials. © 2002 John Wiley & Sons, Inc. J Biomed Mater Res (Appl Biomater) 63: 88–97, 2002; DOI10.1002/jbm.10092

Keywords: acrylic bone cement; 4-N,N dimethylaminobenzyl oleate (DMAO); oleyloxyethylmethacrylate (OMA); physical and mechanical properties

INTRODUCTION

Self-curing cements have been widely used as filling agentsin dentistry and for fixation of joint prosthesis in orthopaedicsurgery. The chemistry of the curing process of these acryliccements is based on the free-radical polymerization of the

methyl methacrylate monomer. The reaction is initiated byfree radicals produced as a result of the decomposition of theinitiator, benzoyl peroxide, in presence of a tertiary amine,usually N,Ndimethyl-4-toluidine (DMT). DMT has beenproved to be a chromosome-damaging agent, as it induceschromosome alterations; it is also a clear inhibitor of proteinsynthesis, interfering with mineralizing processes.1 Some ofthe other undesirable properties associated with self-curingcommercial acrylic bone cements are high peak temperaturesthat result during polymerization of methyl methacrylate andlow fracture toughness. Threshold levels for thermal tissuedamage are as low as 48 to about 60 °C.2

Correspondence to: J. San Roman, Instituto de Ciencia y Tecnologıa de Polımeros,Juan de la Cierva, 3 28006 Madrid, Spain (e-mail: ictsr04@ictp.csic.es)

Contract grant sponsor: Comision Interministerial de Ciencia y Tecnologia; contractgrant number: CICYT (MAT99-1064-C02-01)

© 2002 John Wiley & Sons, Inc.

88

Because of the recognized toxicity of DMT, alternativessuch as the application of higher-molecular-weight tertiaryaromatic amines have been considered.3–8 These types ofcompounds are hydrophobic in nature and therefore lessprone to leaching out from the cured materials. The increasedmolecular weight is also expected to reduce solubility intissue fluids, and thus lessen ease of diffusion into the pulp orother tissues. Among the numerous attempts to reduce themaximum temperature reached during the free-radical poly-merization, one successful method has been to substitutepartially or completely the methyl methacrylate monomerwith a methacrylate ester containing a long alkyl sidechain.9,10 Furthermore, the incorporation of a monomer con-taining a long aliphatic chain is expected to produce anincrease of toughness due to its plastizicing effect. Acrylo-yloxyalkyl esters have been reported to function as plasticiz-ers in poly(vinyl chloride) resins to give easily molded orextruded compounds.11

The present article reports the synthesis of a tertiary aro-matic amine and a high-molecular-weight monomer, bothderived from oleic acid, 4-N,N dimethylamino benzyl oleate(DMAO), and oleyloxyethyl methacrylate, (OMA), respec-tively. Subsequently, the efficacy of the tertiary amine in thecuring of PMMA bone cements was studied by preparingbone cements with the use of DMAO and determining phys-ical properties such as the polymerization exotherm, residualmonomer content, molecular weight of the polymer formed,glass transition temperature, and wettability by contact anglemeasurements. The effect of the incorporation of the mono-mer OMA by part replacement of the monomer, methyl-methacrylate, was also studied, along with DMAO as theactivator. The physical and mechanical properties of theseexperimental cements relevant to their possible uses for or-thopaedic and/or dental applications are reported.

MATERIALS AND METHODS

Materials

Oleoyl chloride (Aldrich), triethyl amine (Merck), and diethylether (Quimicen) were used for the synthesis of 4-N,N di-methylaminobenzyl oleate (DMAO). 4-N,N dimethylaminobenzaldehyde (Fluka) and sodium borohydride (Merck) wereused as received for the preparation of 4-N,N dimethylami-nobenzyl alcohol.12 2-hydroxyethyl methacrylate (HEMA)(Merck) was purified according to the literature13 and usedfor the synthesis of oleyloxyethyl methacrylate. PMMAbeads (33-�m average diameter) were supplied by IndustriasQuirurgicas de Levante (IQL beads); these have already beencharacterized.14 Methyl methacrylate monomer (MMA) sta-bilized with 100 ppm of monomethylether of hydroquinone,benzoyl peroxide (BPO), and N,N dimethyl-4-toluidine(DMT) (all from Merck) were used as received withoutfurther purification. The radiopaque acrylic bone cementPalacos� R (Merck, Darmstadt, Germany) was used for thecontrol formulation.

Synthesis of the Activator 4-N,N Dimethylaminobenzyloleate (DMAO)

Equimolar concentrations of 4-dimethylamino benzyl alco-hol, DMOH, and triethylamine were dissolved in diethylether at room temperature. An equimolar amount of oleoylchloride was added dropwise in a dry atmosphere with con-stant stirring. The reaction was allowed to proceed for 48 h atroom temperature. The reaction medium was filtered to sep-arate the amine chlorhydrate formed and the solute wasconcentrated at reduced pressure, giving the amine DMAO(70% yield).

Synthesis of Oleyloxyethyl Methacrylate (OMA)

Equimolar concentrations of 2-hydroxyethyl methacrylateand catalyst triethyl amine were introduced in a three-neckedflask and dissolved in diethyl ether at room temperature, andan equimolar amount of oleoyl chloride was added dropwiseto the reaction medium in a dry atmosphere with constantstirring. The reaction was allowed to proceed for 48 h at roomtemperature. The reaction medium was filtered off to separatethe amine chlorhydrate formed. An additional amount ofdiethyl ether was added to the product and the mixture wasthen twice washed with 5% (w/v) sodium bicarbonate solu-tion followed by two washings of 5% (v/v) acetic acid solu-tion. The resulting organic phase was dried overnight overmagnesium sulphate and vacuum evaporated (55% yield).

Characterization of DMAO and OMA

DMAO and OMA were characterized by 1H-NMR spectros-copy with the use of deuterated chloroform (5% wt/v) as asolvent and tetramethylsilane (TMS) as an internal standard.The purity of the product was � 98% in both cases.

Acrylic Bone-Cement Formulations

The acrylic bone cement formulations prepared with the oleicacid derivatives are shown in Table I. The commercial bonecement Palacos� R was used as a control. The solid phase ofPalacos� R was used with a modified monomer phase inwhich DMT was substituted with DMAO, in an equimolarconcentration (5.84% wt/wt). Some of the formulations werecarried out in the presence of a radiopaque agent, for exam-ple, ZrO2, because this component is usually included incommercial bone cements to confer radiopacity.

Characterization of Acrylic Bone Cements

The new acrylic bone cements were characterized by mea-suring the polymerization exotherm, dough time, workingand setting time, residual monomer content, glass-transitiontemperature, molecular-weight distribution, mechanical prop-erties, and surface characteristics.

Curing Parameters. The cement in the dough state wasintroduced inside a cylindrical Teflon mold 10 mm in diam-eter and 15 mm high, equipped with a thermocouple con-

89ACRYLIC BONE CEMENTS

nected to a temperature recorder and positioned in the centerof the mold at a height of 3.0 mm in the internal cavity.14

Time was measured from onset of mixing the powder withthe liquid and the temperature recorded. The polymerizationwas carried out at a temperature of 25 °C in all cases. Curingparameters were determined according to the ASTM standardspecification (F451).15

Residual Monomer Content. Residual monomer contentwas determined by means of 1H-NMR spectroscopy with aVarian XL300 spectrometer and calculated with respect toPMMA. Three samples of each type were dissolved in deu-terated chloroform (5% wt/v), with the use of tetramethylsi-lane as internal standard. All the specimens were kept for 7days in air before the analysis.

Thermal Properties of the Cements. Glass transitiontemperatures (Tg) were measured by differential scanningcalorimetry with a Perkin Elmer DSC7 interfaced to a ther-mal-analysis data system TAC 7/DX. The dry samples wereprepared in the form of thin films (15–20 mg) placed inaluminum pans and heated from 30 to 150 °C at a constantrate of 10 °C/min. Tg was taken as the midpoint of the heatcapacity transition.

Molecular Weight Distributions of the Cements. Num-ber- and weight-average molecular weight and polydispersity

were determined by size exclusion chromatography (SEC)(Waters 510 with a refractive-index detector series 200). Aset of 104-, 103- and 500-Å PL-gel columns conditioned at 25°C were used to elute the samples of 10 mg/ml concentrationat 1 ml/min HPLC-grade chloroform flow rate. Calibration ofSEC was carried out with monodisperse standard polystyrenesamples obtained from Polymer Laboratories.

Mechanical Properties. Mechanical properties wereevaluated with the use of a Universal Testing machine (In-stron) with a cell load of 100 KN operating at a crossheadspeed of 1 mm/min at room temperature. Specimens wereprepared according to standard specification ISO 527-1 andtested after storage either in air at room temperature (23�1°C) or saline solution at physiological temperature (37�1 °C)

Figure 1. Scheme of the synthesis of 4-N,N dimethylaminobenzyloleate (DMAO).

Figure 2. 1H RMN spectrum of 4-N,N dimethylaminobenzyl oleate(DMAO) in deuterated chloroform at 25 °C.

TABLE I. Acrylic Bone Cement Formulations Used in this Work

Cement Solid:Liquid PMMA wt% BPO wt% ZrO2 wt% MMA wt% DMAO wt% OMA wt%

DMAO/BPO Molar

Ratio

Cement I 2:1 PALACOS� R 94.16 5.84 — 2Cement II 2:1 84.11 0.89 15 94.16 5.84 — 2Cement III 2:1 84.11 1.5 15 94.16 5.84 — 1Cement IV 2:1 98.5 1.5 — 97.43 2.57 — 0.5Cement V 1.8:1 98 2 — 84.16 5.84 10 1Cement VI 1.8:1 98 2 — 79.16 5.84 15 1Cement VII 1.5:1 98 2 — 89.16 5.84 5 1Cement VIII 1.5:1 98 2 — 84.16 5.84 10 1Cement IX 1.5:1 98 2 — 79.16 5.84 15 1Cement X 1.8:1 88 2 10 84.16 5.84 10 1Cement XI 1.5:1 88 2 10 84.16 5.84 10 1

90 VAZQUEZ ET AL.

for 5 weeks. A minimum of six specimens was tested for eachbatch. Representative fracture surfaces from the dry and wettensile specimens of the different cements were also analysedby scanning-electron microscopy (SEM). Each specimen wassputter covered with gold to render the surface electricallyconducting.

Surface Characteristics. The contact angle measure-ments were performed on dry films of cement with the use ofthe Contact Angle Measuring System G10 (Kruss). The sur-face free energy was calculated by the approach introducedby Fowkes, in which the total surface tension is considered asa sum of independent terms, each representing a particularintermolecular force,16 and by the application of the equationof Owens and Wendt,17 which is an extension to a so-calledpolar component. The liquids used for this purpose weremethylene iodide and distilled water.17 The dispersion-forcecomponent and the polar-force component of the surface

energy of water are 21 and 51 mN/m, respectively,17 and thedispersion-force component of the methylene iodine is 50mN/m.17

Statistical Analysis. The data obtained for the mechani-cal properties were analyzed with the use of a one-wayANOVA test and the Students–Newman–Keuls method. A

Figure 3. Scheme of the synthesis of the monomer oleyloxyethylmethacrylate (OMA).

Figure 4. 1H RMN spectrum of oleyloxyethyl methacrylate (OMA) indeuterated chloroform at 25 °C.

TABLE II. Assignment of the Resonance Signals Appeared in the 1H NMR Spectra of DMAO and OMA, Respectively

DMAO OMA

Resonance Signal Assignment Resonance Signal Assignment2.94 ppm (CH3)2NO 6.11 and 5.56 ppm OCH2A7.22 and 6.69 ppm Aryl protons 1.92 ppm O(CH3)�

4.99 ppm OCH2OOCO 4.31 ppm OOOCH*2OCH*2OOO2.28 ppm OOCOOCH*2O 2.30 ppm OOCOOCH*2O1.60 ppm OOCOOCH2OCH*2O 1.59 ppm OOCOOCH2OCH*2O1.98 ppm OCH*2OCHACHOCH*2O 1.98 ppm OCH*2OCHACHOCH*2O5.32 ppm OCH*ACH*O 5.32 ppm OCH*ACH*O1.15–1.43 ppm O(CH2)4O and O(CH2)6O 1.04–1.45 ppm O(CH2)4O and O(CH2)6O0.86 ppm OCH3 0.85 ppm OCH3

* Indicates the hydrogen considered.R: OCH2OCH2O(CH2)4OCH2OCHACHOCH2O(CH2)6OCH3

91ACRYLIC BONE CEMENTS

significance level of p�0.05 was considered. The contactangle results were analyzed with the use of one way ANOVAat a significance level of p�0.05.

RESULTS AND DISCUSSION

Characterization of 4-N,N Dimethylaminobenzyloleate (DMAO)

4-N,N dimethylaminobenzyl oleate (DMAO) was synthe-sized in good yields (�70%) by reacting DMOH12 and oleoylchloride at room temperature in the presence of triethyl amineas a catalyst (Figure 1). The compound DMAO was purifiedprior to characterization. The 1H NMR spectrum is shown inFigure 2 and peak assignments are presented in Table II. Theesterification of the alcohol group present in DMOH was

confirmed by the appearance of the peak at 4.99 ppm due tothe resonance of the —CH2—O—COR protons, which shiftdownfield because of the proximity of the ester group. As theoleoyl chloride reacts to form the ester, the signals due to theprotons in close proximity to the chlorine atom suffer aupfield shift and peaks are observed at 2.30 and 1.59 ppm,respectively, because of the resonance of the —CH2*—COO— and CH2*—CH2—COO— protons in the spectrumof DMAO. The rest of the protons of the oleoyl hydrocar-bonated chain in the DMAO molecule appear at the samechemical shift as those in the spectrum of oleoyl chloride andare assigned by comparison with those of that spectrum.

Characterization of Oleoyloxyethyl Methacrylate (OMA)

OMA was synthesized by esterification of oleoyl chloridewith 2-hydroxyethyl methacrylate (HEMA) at room temper-ature in the presence of triethylamine as catalyst (Figure 3).This esterification method was used in contrast to that pat-ented by Monsanto�,11 as it is a simple one-step procedureand does not require the application of heat or strong acids.Figure 4 shows the 1H NMR spectrum of the purified com-pound and the assignment of the signals are shown in TableII. A single resonance peak at 4.3 ppm in the OMA spectrumconfirms the esterification of the methylol protons in HEMAas the two resonance peaks due to the —CH2OH and—OCH2— are replaced. The rest of the signals have beenassigned by comparison with the corresponding signals in thelong chain of the oleoyl chloride.

Application of DMAO in Curing Bone Cements

The polymerization reaction was followed by measuring theincrease in temperature with time.18 Figure 5 shows therepresentative time–temperature profiles of several cementformulations cured with the system DMAO/BPO under con-ditions described in the experimental section. As is estab-lished, the amine molecules activate decomposition of BPO,

Figure 5. Exotherms of polymerization of PMMA cements preparedwith the DMAO/BPO redox system along with that of the commercialacrylic bone cement Palacos� R. Compositions of the cements aregiven in Table I.

TABLE III. Values of Residual Monomer Content and Curing Parameters Registered at 25 °C of Several Acrylic Bone CementFormulations Prepared with DMAO as Activator and in the Presence of OMA in the Liquid Phase

Cement Solid:Liquid Amine/BPOtdough

(min) (sd)tworking

(min) (sd)tsetting

(min) (sd)Tpeak

(°C) (sd)Residual

Monomer (%)

PALACOS� R 2:1 2 2.0 [0.12] 9.0 [0.14] 11.0 [0.27] 73 [0.98] 3.60 [0.19]Cement I 2:1 2 2.0 [0.20] 16.0 [0.25] 18.0 [0.02] 53 [0.20] 3.70 [0.05]Cement II 2:1 2 5.5 [0.20] 11.5 [0.25] 17.0 [0.04] 54 [0.15] 4.40 [0.59]Cement III 2:1 1 4.5 [0.30] 11.5 [0.35] 16.0 [0.17] 62 [0.40] 3.50 [0.40]Cement IV 2:1 0.5 3.0 [0.25] 14.0 [0.25] 17.0 [0.10] 57 [0.49] 3.50 [0.30]Cement V 1.8:1 1 7.0 [0.55] 12.0 [0.15] 19.0 [0.35] 61 [1.20] 1.84 [0.26]Cement VI 1.8:1 1 8.0 [0.20] 10.5 [0.30] 18.5 [0.70] 53 [1.00] 1.63 [0.27]Cement VII 1.5:1 1 10.5 [0.50] 10.5 [0.30] 21.0 [0.92] 64 [1.50] 2.36 [0.36]Cement VIII 1.5:1 1 10.2 [0.45] 13.0 [0.75] 23.0 [0.35] 60 [1.70] 1.60 [0.42]Cement IX 1.5:1 1 8.0 [0.55] 14.0 [1.18] 22.0 [1.25] 55 [1.00] 1.50 [0.57]Cement X 1.8:1 1 8.0 [0.55] 10.0 [0.70] 18.0 [0.25] 59 [1.50] 1.10 [0.17]Cement XI 1.5:1 1 15.0 [1.00] 12.0 [0.29] 27.0 [1.30] 59 [0.40] 1.38 [0.39]

92 VAZQUEZ ET AL.

generating benzoate free radicals as well as aminomethyl freeradicals, which can also initiate radical polymerization. Thepolymerization proceeds with a gradual increase in tempera-ture for a certain time and then undergoes a marked acceler-ation due to the well-known gel effect; hence a peak temper-ature is observed. The time–temperature curves showed alower peak temperature with the novel activator and a reduc-tion in 10–15 °C was observed depending on the amine/BPOmolar ratio in comparison with traditional DMT. The settingtime was around 6–7 min higher than that obtained withDMT. Similarly, when DMAO was used to cure the com-mercial bone cement Palacos� R, (CEMENT-I) a decrease of20 °C in the peak temperature and an increase in setting timeof 7 min was observed. The increase in setting time with thenew activator is attributed to the longer diffusion time of theamine bearing a long aliphatic chain in the para position.19

This fact was also observed with the amine derived fromlauric acid, and it was confirmed by the fact that setting timedecreased with increasing polymerization temperature.8

When formulations were prepared with an amine/BPO molarratio of 1 (CEMENT-III) a decrease in setting time and a risein the maximum temperature were obtained; however, peaktemperature remained 10 °C lower than that obtained with thecontrol. With a molar ratio of 0.5 (CEMENT-IV) a slightincrease in setting time was obtained and the optimumDMAO/BPO molar ratio20 was taken as 1 for further exper-iments. The application of DMAO is beneficial over conven-tional DMT as leaching into viscous blends or molding com-pounds will be minimized because of the greater molecularsize of DMAO (three times higher than that of DMT) and the

presence of the unsaturated group, which would allow it to beincorporated chemically into the final matrix. Values of re-sidual monomer for any formulation were in the range of3.5–4.5wt%, indicating that the amount of unpolymerizedmonomer was not higher than in commercial formulationscured with DMT.

Partial Substitution of Methylmethacrylate Monomerwith Oleoyloxyethylmethacrylate

The presence of a high-molecular-weight monomer such asoleyloxyethyl methacrylate (OMA) is expected to reduce themaximum temperature reached during the polymerization ofacrylic bone cements,9,10 The setting of these cements wasinitiated by the DMAO/BPO redox system with an amine/BPO molar ratio of 1, as established previously. The solid:liquid ratio was altered because of the higher viscosity of thenew monomer. Table III shows the curing parameters of theOMA-containing cements. Values of dough time for theseformulations were higher than that permitted by the standard,e.g., 5 min, because of the decrease in the solid:liquid ratioused in the preparation of these cements. A decrease in peaktemperature with increasing OMA concentration was ob-served, as expected. A reduction of the solid:liquid ratiomainly provided an increase in setting time, giving rise tovalues of 27 min for a radiopaque formulation prepared witha solid:liquid ratio of 1.5:1 and containing 10 wt% OMA. Fora solid:liquid ratio of 1.8:1 the presence of 15 wt% of the newmonomer provided an increase of setting time in approxi-mately 3 min, and a decrease of peak temperature in 10 °C.A similar behavior was found by Brauer, Steinberger, andStansbury,9 using partially substituted MMA by a high-mo-lecular-weight methacrylate such as dicyclopentenyloxyethylmethacrylate. The cements formulated with the solid:liquidratio of 1.5:1 showed similar setting kinetics. The workingtimes21 of the cements are shown in Table III; the use ofDMAO in conjunction with OMA provided an increase inworking time, which is desirable for the implantation of thedough mass into the femoral cavity. Values of residual mono-mer content for the cements modified in the liquid phase(Cements V–XI) were lower compared to those for CementsI–IV, which can be attributed to the higher mobility of thereacting species.

TABLE IV. Values of Glass-Transition Temperature (Tg) andNumber- and Weight-Average Molecular Weights (Mn and Mw,Respectively) and Polydispersity of Radiolucent CementsFormulated with OMA

Cement Tg (°C) Mn � 10�5 Mw � 10�5 Polydispersity

Cement V 85 1.3 3.4 2.65Cement VI 85 1.1 3.3 2.96Cement VII 86 1.2 3.8 3.23Cement VIII 85 1.1 3.6 3.19Cement IX 81 1.2 3.7 3.04

TABLE V. Mechanical Properties of Bone Cements Formulated with DMAO Tested after 5 Weeks of Storage in Air or Saline Solution

Cement Storage MediumCompressive Strength

(MPa) [sd]Ultimate Tensile

Strength (MPa) [sd]Young’s Modulus

(GPa) [sd]Strain to Failure

(%) [sd]

PALACOS� R AIR 81.1 [3.7] 42.1 [3.06] 3.58 [0.30] 2.6 [0.59]Cement II AIR 85.9 [3.4] 42.6 [2.46] 1.60 [0.06] 5.4 [0.37]Cement III AIR 85.6 [2.2] 41.2 [2.27] 1.54 [0.14] 5.0 [0.27]Cement IV AIR 78.1 [2.9] 41.9 [1.70] 1.35 [0.07] 5.0 [0.12]PALACOS� R 0.9% NaCl 71.4 [1.0] 37.9 [2.48] 2.40 [0.06] 5.2 [0.34]Cement I 0.9% NaCl 76.3 [2.6] 36.3 [1.07] 1.20 [0.05] 6.3 [0.24]Cement II 0.9% NaCl 73.0 [3.0] 37.9 [0.07] 1.20 [0.09] 5.6 [0.55]Cement III 0.9% NaCl 75.6 [2.6] 37.8 [1.98] 1.26 [0.06] 5.3 [0.17]Cement IV 0.9% NaCl 71.8 [2.9] 39.0 [1.26] 1.48 [0.14] 5.4 [0.40]

93ACRYLIC BONE CEMENTS

Thermal Properties and Molecular Weight

The effect on the thermal properties due to the inclusion ofDMAO and OMA in the cement matrix were determined bymeasuring the glass-transition temperature (Tg). Recently, Tg

has been used for additional characterization of bone ce-ments22 because it is related to the flexibility and toughnessof the cured biomaterials. It has been postulated that materialswith too high Tg are brittle in nature, which is indirectlyrelated to failure of the cement and subsequently the loosen-ing of the components. Values of Tg of the cements formu-lated with both oleic acid derivatives are shown in Table IV.PMMA cements prepared with the beads used in this studyand cured with DMT exhibited a Tg of �111 °C,18 whereasPalacos� R had a Tg of 93 °C due to the presence ofmethylacrylate units in the prepolymer beads. Cements II–IVbased on PMMA and formulated with DMAO, present valuesof Tg in the range 90–95 °C, showing a decrease in Tg by

Figure 6. SEM microphotographs of representative fracture surfaces of (a) Cement IV and (b)Cement III stored in air for 5 weeks. Original magnification 90�.

TABLE VI. Tensile Properties of the Acrylic CementsFormulated with Fatty Acid Derivatives after 5 Weeks ofStorage in Air

Cement

UltimateTensileStrength

(MPa) [sd]Young’s Modulus

(GPa) [sd]

Strain toFailure

(%) [sd]

PALACOS� R 42.1 [3.1] 3.58 [0.30] 2.60 [0.59]Cement V 39.4 [2.3] 3.13 [0.41] 2.17 [0.43]Cement VI 34.9 [1.7] 2.69 [0.47] 1.96 [0.77]Cement VII 48.7 [2.2] 3.65 [0.90] 2.06 [0.10]Cement IX 34.0 [5.3] 3.56 [0.66] 1.92 [0.34]Cement X 37.5 [1.5] 3.26 [0.60] 2.44 [0.80]Cement XI 34.9 [1.2] 2.69 [0.47] 1.96 [0.77]

94 VAZQUEZ ET AL.

15–20 °C than the corresponding cement cured with DMT(Tg�111 °C),18 which indicated the plasticizing effect of theoleic chain in the activator. Cements formulated with OMAexhibited Tgs around 85 °C (Table IV), which can be asso-ciated with the increase in the oleic residues belonging toboth activator and monomer.

It is a well-established principle in polymer science thatmolecular weight and molecular distributions are useful in-dicators of mechanical properties, mainly fatigue strengths.23

The molecular weight of the hardened cement matrix dependson different parameters, among them the molecular weight ofthe monomers in the liquid phase. The introduction of a newactivator and subsequently a new monomer is expected toinfluence the molecular weight of the resultant polymer.Thus, molecular weight distributions of the cements formu-lated with the oleic derivatives were determined. Values ofthe number- and weight-average molecular weight are shownin Table IV. Number-average molecular weights of thesecements were higher than that of the prepolymerized PMMAbeads used in this study (Mn�6.4�104).18 This fact can beattributed not only to the presence of a relatively high-molecular-weight monomer (OMA) in the liquid phase, butalso to the Trommsdorff effect, characteristic of the bulkpolymerization of acrylates and methacrylates in which the

reaction rate increases as a consequence of the reduction inthe termination reaction of the macro radicals because of anincrease in viscosity of the medium.24 However, the molec-ular weights obtained are in the range of those reported forcommercially available cements.25

Effect on Mechanical Properties

The effect of the fatty-acid amino derivative on the mechanicalproperties, such as tensile and compressive strengths, are shownin Table V. Bone cements formulated with the new activatorexhibited comparable tensile strengths to that of Palacos� R.However, the mean values of compressive strengths were foundto be significantly different at levels p�0.001 and a pairwisecomparison using a Student–Newman–Keuls test indicated thatthe Cements II and III had greater compressive strengths thanPalacos� R. The differences in the mean values of Young’smodulus showed that there was a significant decrease (p�0.001)in the experimental cements in comparison to Palacos� R andthe strain to failure exhibited a significant increase (p�0.001) incomparison to Palacos� R. The increase in strain to failure andlowering in modulus can be attributed to the plastizicing effectof the long aliphatic chain of the activator, both of which arebeneficial in reducing the brittleness of the cement. Mechanicalproperties of specimens conditioned in saline solution for 5weeks were evaluated in order to mimic the physiological envi-ronment, and results followed the same trend as those obtainedin dry specimens. Compressive strengths of the experimentalcements were significantly (p�0.001) higher than the controlexcept for the cement containing the lower concentration ofDMAO (Cement IV). A comparison of the ultimate tensilestrengths showed that the incorporation of the amine DMAO inboth Palacos� R and in the experimental cement (Cement II) didnot produce any statistically significant difference (p�0.001).Young’s modulus showed a significant decrease (p�0.001) incomparison to Palacos� R and the strain to failure values werenot statistically significant.

Cement VII, containing the new initiator DMAO andOMA (5% by wt), exhibited significantly superior mechani-cal properties in comparison to Palacos� R. However, higherconcentrations of OMA monomer decreased tensile strength(statistically significant at p�0.001). The Young’s modulusand strain to failure for Cement VII was not statisticallydifferent from that of the control; thus low concentrations ofthe OMA monomer enhanced tensile strength without impair-ing the other two parameters.

SEM was conducted to examine the fracture surfaces ofthe different formulations stored either in air or in salinesolution. The formulation containing the lowest concentrationof DMAO (2.57 wt%) stored in air showed a relatively brittlefracture surface, although when 5.84 wt% of DMAO wasadded to the liquid phase, a slightly more ductile fracture wasobserved. Both fracture surfaces are shown in Figure 6. Theaspect of the fracture surface of Cements II and III stored insaline for 5 weeks are shown in Figure 7, both exhibiting aductile fracture, which resembles that of Cement III stored inair.

Figure 7. SEM microphotographs of representative fracture sur-faces of (a) Cement II and (b) Cement III, after conditioning in saline(37�1 °C) for 5 weeks. Original magnification 90�.

95ACRYLIC BONE CEMENTS

Surface Characteristics

Wettability of the modified cements was studied because ofthe importance of the interactions of biological species withthe surface of solid substrate in the implantation of a bioma-terial device. The introduction of hydrophobic moleculeswithin the matrix is expected to influence wettability of thematrix. A method for approximating the surface free energyof solids has been developed by Owens and Wendt.17 Itdepends simply on the measurement of contact angle withtwo liquids of very different polarity, that is, methyleneiodide and water.17 This method has recently been applied tomodified formulations of acrylic bone cements.26 Values ofcontact angle for the cements prepared in this work are givenin Table VII. The contact angles of the cements with waterwere higher than those when measured with methylene io-dide, which indicated the hydrophobic nature of the poly-meric surfaces. The cement formulated with the lowestDMAO content gave contact–angle values similar to thoseobtained with DMT, indicating that the surface characteristicsdid not appreciably change for this DMAO concentration.However, with a DMAO content of 5.84 wt%, an increase inthe hydrophobicity of the surface was observed as is reflectedby the significant (p�0.001) decrease in the contact anglewith methylene iodide. This increase in hydrophobicity arisesfrom the presence of the fatty-acid residues present in theactivator molecules. Total surface free energy did not appre-ciably change because of the compensation of both polar anddispersive components.

CONCLUSIONS

Derivatives of oleic acid, DMAO, and OMA were synthe-sized and successfully used in the formulation of acrylic bonecements. DMAO, when used as the activator, lowered themaximum temperature reached during polymerization, whichis beneficial in orthopaedic bone cements. Mechanical prop-erties were not compromised when both activators and mono-mers derived from oleic acid were introduced in the bone-cement formulation. Furthermore, low concentrations of themonomer, OMA provided an enhancement of tensilestrength. However, a study of mechanical properties such asfracture toughness and fatigue life is necessary before the invivo application of these cements.

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TABLE VII. Surface Properties of Radiolucent Bone Cements Formulated with the Novel Activator: Values of Contact Angle (�),Values of Surface Energy of Solid (�s) and those of its Disperse (�s

d) and Polar (�sp) Components

Cement

ActivatorConcentration

(wt%) � Water� Methylene

Iodide�s

(mN/m)�s

d

(mN/m)�s

p

(mN/m)

Control 1.9 (DMT) 77 [2.8] 40 [3.3] 44.2 40 4.2Cement II 5.84 (DMAO) 79 [2.6] 33 [1.8] 46.6 43 3.6Cement III 5.84 (DMAO) 79 [1.1] 34 [1.6] 45.2 43 3.2Cement IV 2.57 (DMAO) 78 [2.3] 40 [2.4] 44.0 39 4.0

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