modified acrylic bone cement with high amounts of ethoxytriethyleneglycol methacrylate

11
Biomaterials 20 (1999) 453 463 Modified acrylic bone cement with high amounts of ethoxytriethyleneglycol methacrylate B. Pascual!, M. Gurruchaga!, M.P. Ginebra", F.J. Gil", J.A. Planell", B. Va´zquez#, J. San Roma´n#, I. Gon 8 i!,* !Dpto. Ciencia y Tecnologı & a de Polı & meros, Facultad de Quı & mica, UPV/EHU, Apdo 1072, 20080 San Sebastia & n, Spain "ETSII, UPC. Av. Diagonal 647, 08028 Barcelona, Spain #Instituto de Ciencia y Tecnologı & a de Polı & meros, CSIC, Juan de la Cierva, 3, 28006 Madrid, Spain Received 5 January 1998; accepted 1 September 1998 Abstract One cause of arthroplasty failure is the brittle mechanical behavior of bone cements. However, the improvement of cement formulations must also be accompanied by the maintenance of a wide variety of characteristics. New bone cements were obtained by the substitution of high percentages, up to 60% (v/v), of methyl methacrylate (MMA) by a higher molecular weight and more hydrophilic monomer, ethoxytriethyleneglycol methacrylate (TEG). The essential advantages of these materials were the decrease of maximum temperature together with a decrease in the residual monomer content with respect to conventional cement formulations. The water absorption process obeyed diffusion laws and the equilibrium water content increased by the introduction of higher percentages of the hydrophilic component. This characteristic had an appreciable effect on the viscoelastic behavior analyzed by DMTA. These modified bone cements had reduced polymerization shrinkage and similar levels of porosity. Tensile test revealed that the introduction of TEGMA gave rise to an important modification of the mechanical behavior, with a noticeable increase in the fracture strain. This fact was also confirmed by means of the analysis of the fracture surfaces by SEM. ( 1999 Elsevier Science Ltd. All rights reserved Keywords: Bone cement; Water absorption; Polymerization shrinkage; Mechanical properties 1. Introduction Poly(methyl methacrylate) (PMMA) is used in a wide variety of medical and dental applications and is current- ly the only material for anchoring cemented arthroplas- ties to the contiguous bones. In this application the main functions of the cement are to transfer body weight and service loads from the prosthesis to the bone, and in- crease the load carrying capacity of the prosthesisbone cementbone system. However, despite the relatively good success rate of implant fixation with acrylic-based bone cement, a number of persistent problems are en- countered. These disadvantages are related mainly to the necrosis associated to the use of the cement and the relatively poor mechanical behavior of the cement [1]. * Corresponding author. Fax: 0034 43 212236; e-mail: popgoeci@ sq.ehu.es Thermal necrosis had been claimed to be a possible factor leading to later failure of cemented arthroplasties [2, 3]. It is postulated that the cement has a role in thermal necrosis of bone, impaired local blood circula- tion and predisposition to membrane formation at the cementbone interface [4]. All these phenomena have been attributed to the high exothermic temperature of the cement, amounting to about 67124°C, depending on the cement formulation [5]. The temperature increase is governed by the amount of polymerizing monomer and the chemical composition of the cement components [6, 7]. Moreover, the chemical necrosis has also been postulated to be due to the release of unreacted monomer [8]. Hence, it could be of clinical importance to reduce the residual monomer content, without influencing ce- ment quality. On the other hand, either the shrinkage of cement during polymerization [9] or the poor mechanical be- havior of the cement contribute to the aseptic loosening 0142-9612/99/$ see front matter ( 1999 Elsevier Science Ltd. All rights reserved. PII: S 0 1 4 2 - 9 6 1 2 ( 9 8 ) 0 0 1 9 1 - 4

Upload: b-pascual

Post on 02-Jul-2016

215 views

Category:

Documents


1 download

TRANSCRIPT

Page 1: Modified acrylic bone cement with high amounts of ethoxytriethyleneglycol methacrylate

Biomaterials 20 (1999) 453—463

Modified acrylic bone cement with high amountsof ethoxytriethyleneglycol methacrylate

B. Pascual!, M. Gurruchaga!, M.P. Ginebra", F.J. Gil", J.A. Planell",B. Vazquez#, J. San Roman#, I. Gon8 i!,*

!Dpto. Ciencia y Tecnologı&a de Polı&meros, Facultad de Quı&mica, UPV/EHU, Apdo 1072, 20080 San Sebastia& n, Spain"ETSII, UPC. Av. Diagonal 647, 08028 Barcelona, Spain

#Instituto de Ciencia y Tecnologı&a de Polı&meros, CSIC, Juan de la Cierva, 3, 28006 Madrid, Spain

Received 5 January 1998; accepted 1 September 1998

Abstract

One cause of arthroplasty failure is the brittle mechanical behavior of bone cements. However, the improvement of cementformulations must also be accompanied by the maintenance of a wide variety of characteristics. New bone cements were obtained bythe substitution of high percentages, up to 60% (v/v), of methyl methacrylate (MMA) by a higher molecular weight and morehydrophilic monomer, ethoxytriethyleneglycol methacrylate (TEG). The essential advantages of these materials were the decrease ofmaximum temperature together with a decrease in the residual monomer content with respect to conventional cement formulations.The water absorption process obeyed diffusion laws and the equilibrium water content increased by the introduction of higherpercentages of the hydrophilic component. This characteristic had an appreciable effect on the viscoelastic behavior analyzed byDMTA. These modified bone cements had reduced polymerization shrinkage and similar levels of porosity. Tensile test revealed thatthe introduction of TEGMA gave rise to an important modification of the mechanical behavior, with a noticeable increase in thefracture strain. This fact was also confirmed by means of the analysis of the fracture surfaces by SEM. ( 1999 Elsevier Science Ltd.All rights reserved

Keywords: Bone cement; Water absorption; Polymerization shrinkage; Mechanical properties

1. Introduction

Poly(methyl methacrylate) (PMMA) is used in a widevariety of medical and dental applications and is current-ly the only material for anchoring cemented arthroplas-ties to the contiguous bones. In this application the mainfunctions of the cement are to transfer body weight andservice loads from the prosthesis to the bone, and in-crease the load carrying capacity of the prosthesis—bonecement—bone system. However, despite the relativelygood success rate of implant fixation with acrylic-basedbone cement, a number of persistent problems are en-countered. These disadvantages are related mainly to thenecrosis associated to the use of the cement and therelatively poor mechanical behavior of the cement [1].

*Corresponding author. Fax: 0034 43 212236; e-mail: [email protected]

Thermal necrosis had been claimed to be a possiblefactor leading to later failure of cemented arthroplasties[2, 3]. It is postulated that the cement has a role inthermal necrosis of bone, impaired local blood circula-tion and predisposition to membrane formation at thecement—bone interface [4]. All these phenomena havebeen attributed to the high exothermic temperature ofthe cement, amounting to about 67—124°C, depending onthe cement formulation [5]. The temperature increase isgoverned by the amount of polymerizing monomer andthe chemical composition of the cement components[6, 7]. Moreover, the chemical necrosis has also beenpostulated to be due to the release of unreacted monomer[8]. Hence, it could be of clinical importance to reducethe residual monomer content, without influencing ce-ment quality.

On the other hand, either the shrinkage of cementduring polymerization [9] or the poor mechanical be-havior of the cement contribute to the aseptic loosening

0142-9612/99/$ — see front matter ( 1999 Elsevier Science Ltd. All rights reserved.PII: S 0 1 4 2 - 9 6 1 2 ( 9 8 ) 0 0 1 9 1 - 4

Page 2: Modified acrylic bone cement with high amounts of ethoxytriethyleneglycol methacrylate

Fig. 1. Mechanism of TEG synthesis.

of the implant. The cement produces stress shielding ofthe proximal femur, with a high proportion of the jointforce being transmitted from stem to bone distally. Thissituation becomes worse with time as progessive resorp-tion of proximal bone leads to a diminishing transfer ofload to this part of the femur, and a consequent increasein distal load transfer [10]. Thus, from clinical studies wecan conclude that bone cement failure, either at theinterfaces or through the cement is one of the majorcauses of implant failure [1, 11, 12]. PMMA is a glassyamorphous thermoplastic polymer which is known to beof low strength brittle material at body temperature. Theincorporation of polymers or copolymers is reported toimprove some physical properties of cements comparedwith conventional material, namely setting time andearly strength [13]. Several attempts to improve themechanical properties of bone cements have been carriedout in recent years [14]. In this sense, the use of a morehydrophilic monomer in the liquid phase may introducesignificant improvements in toughness of the cement. Themechanical properties of acrylic bone cements are in-fluenced by the absorption of low molecules mass speciesfrom the environment surrounding the bone cement.Therefore, is to be expected that the fracture toughness ofthe material could be improved by the plasticizing effectso introduced.

Thus, taking into account the above considerationsand the promising results obtained with the addition oflow amounts of ethoxytriethyleneglycol methacrylate(TEG) [15] to cement formulations ((20% in volume),the aim of the present investigation was to evaluate theeffect of the addition of higher percentages of TEG. Inthis sense, the influence on the mechanical behavior wasanalyzed as well as on the setting characteristics andresidual monomer content. In the same way, any applica-tion of bone cements requires knowledge of their dimen-sional stability [16, 17, 18]. Polymerization shrinkageduring setting and subsequent dimensional changes as

a consequence of water absorption in body conditionscritical to its long-term stability in vivo, are studied.

2. Materials and methods

2.1. Materials

Methyl methacrylate (MMA) (Merck) stabilized with100 ppm of hydroquinone monomethylether was used asreceived. The N,N dimethyl-4-toluidine (DMT) (Merck)was used as activator of the polymerization reaction.

The monomer ethoxytriethyleneglycol monomethac-rylate (TEG) was synthesized in our laboratory [19].Methacryloyl chloride (Fluka), triethylamine (Merck),triethyleneglycol monoethyl ether (Fluka), and dichloro-methane (Panreac) were used in the preparation of thismonomer. All of them were reagent grade and used asreceived. To synthesize this monomer the method of theesterification of acyl chlorides was applied. The methac-ryloyl chloride was treated with triethyleneglycol mono-ethyl ether in the presence of triethylamine. The mostprevalent reaction mechanism followed is by far the tet-rahedral mechanism, shown in Fig. 1.

The solid component consisted of PMMA beads witha particle size distribution of 10—60 lm, an average dia-meter of DM "33.10 lm and a molecular weight ofMM

8"130]103 (Plexigum M339 (Rhom)). Benzoyl per-

oxide (BPO) (Merck) was used as initiator after its purifi-cation by recrystallization in methanol.

2.2. Methods

2.2.1. Specimen preparationThe study of the setting reaction in cements with

PMMA beads of different sizes showed high temper-atures in bone cements formulated with PMMAbeads Plexigum M339 [20]. In addition to this, the

454 B. Pascual et al. / Biomaterials 20 (1999) 453—463

Page 3: Modified acrylic bone cement with high amounts of ethoxytriethyleneglycol methacrylate

copolymerization of TEG/MMA at low conversion waspreviously investigated and the reactivity rates weredetermined [19]. It was found that TEG is a less reactivemonomer than MMA and, therefore, would give rise toa slower rate of polymerization. Thus, it could be ex-pected as a reduction in the peak temperature by theaddition of TEG to bone cements formulated withPlexigum M339 as solid component. This featurecould be advantageous because of the possible reductionof the thermal necrosis associated with the use of bonecements.

Therefore, the modified bone cements were formulatedwith the introduction of a more hydrophilic monomer inthe liquid component. In this sense, MMA was partiallysubstituted by TEG ranging from 10 to 60% (v/v). A typi-cal solid/liquid ratio of 2/1 was used. The liquid compo-nent consisted of a 1% (v/v) DMT in monomer solutionin all cases. The solid component consisted of PlexigumM339 beads to which the corresponding amount of finelyground BPO (1.25% w/w) was added.

The preparation of acrylic bone cement was carriedout following the method used for classical bone cementdescribed in ASTM standard (F451-86). The componentsof bone cements were hand-mixed and when the doughstate was reached the mass was placed in the correspond-ing mold and allowed to cure for 1 h at 37°C.

2.2.2. CharacterizationThe variation of the mass temperature with time was

registered using a thermocouple connected to a high-sensitivity thermotester. A cylindrical Teflon mould spe-cifically designed by our group to obtain reproducibledata at a working temperature of 37°C was used [20].The residual monomer content of the cements [21], poly-merization shrinkage as a function of the density [22, 23]and sorption kinetics in water and in saline solution wereevaluated as in previous papers [20].

Dynamic mechanical analysis (DMTA) was carriedout in bending mode from 20 to 200°C by means ofDMTA MKIII instrument. The specimen size was:40 mm]10 mm]1.5 mm, at a frequency of 1 Hz withdisplacement of 0.064 mm and a heating rate of3°C min~1 [18]. Glass transition temperature of the ce-ments was read off as the temperature at which the lossmodulus or loss factor passed through a maximum.

In clinical service the prosthesis is subjected to static orquasistatic direct compressive forces during certain activ-ities, such as in one-legged stance [24]. Also, the cementmantle has been postulated to be a compressive wedgebetween the femoral stem and the bone, by acting asa shock absorber between the implant and the bone [25].Thus, quasistatic compressive properties of the bone ce-ment are relevant. In vitro tests have been carried out inaccordance with ASTM (F451) [26] in a Instron elec-tromechanical testing machine at 22 mmmin~1. Tensilestresses are observed in various parts of an arthroplasty,

for example, on the lateral side of a hip implant, due tobending. Thus, quasistatic tensile properties of the ce-ment are important [14]. The tensile test specimens wereprepared according to the standard specification ISO527-1 [27] and BSI B5278 sample 1BA. The tests werecarried out in a servohydraulic MTS Bionix 858 testingmachine with a cross-head speed of 1 mmmin~1. Thesamples were maintained at 37°C in saline solution untilequilibrium uptake was attained.

Representative fracture surfaces from the tensile speci-mens were analyzed by scanning electron microscopy(SEM) using a JEOL JSM-6400 microscopy operating at20 kN. Each specimen was cut approximately 1 cm belowthe fracture surface, then covered with gold to render thesurface electrically conducting.

3. Results and discussion

3.1. Polymerization kinetics

Polymerization kinetics of modified bone cementswere studied by recording the time dependence of poly-merization temperature of bone cements, and their per-formance with ASTM standard was evaluated [28].

The temperature—time curves (Fig. 1) registered duringpolymerization of bone cements modified with differentamounts of TEG showed great influence of the additionof this monomer. Obviously, data followed the sametendency observed before with bone cements modifiedwith low amounts of TEG [15], and the important de-crease in the maximum temperature reached at longertimes was noticeable. The evolution of the most represen-tative curing parameters, peak temperature and settingtime, with the addition of higher percentages of TEG isplotted in Fig. 2. When the cement was prepared with60% (v/v) of TEG, the peak temperature decreasedlinearly in approximately 40°C (r"0.991) and enlargedthe setting in 8 min with an exponential tendency(r"0.988).

Actually, polymerization of vinyl monomers involvespractically the release of almost the same amount of heat[29]. However, the achieved peak temperature dependson the total polymerization heat, polymerization kineticsand heat transport phenomenon [6]. The decrease in themaximum temperature of modified bone cements can beattributed to all factors. First, as has been mentionedbefore, the study of TEG/MMA copolymers shows lesserreactivity of TEG with respect to MMA and, therefore,involves a slow down of the copolymerization reaction.In addition to this, the introduction of a monomer witha higher molecular weight implies a decrease in the totalmols of monomer because of maintaining the total reac-tion volume. Thus, the lesser amount and slow release ofpolymerization heat during the setting reaction allow thegradual dissipation of heat through the mass, leading to

B. Pascual et al. / Biomaterials 20 (1999) 453—463 455

Page 4: Modified acrylic bone cement with high amounts of ethoxytriethyleneglycol methacrylate

Fig. 2. Polymerization exotherm of modified bone cements obtainedwith differents percentages (v/v) of TEG: (d) 0%; (s) 10%; (m) 30%;(]) 40%; (h) 60%.

a lower temperature. This feature is of great importancesince the slower rise in temperature has the advantagethat the heat generated can be dissipated more easilyfrom the material during setting, causing less adverseeffect on the surrounding tissues.

The increase of setting time with the addition of higherpercentages of TEGMA to bone cement formulation isplotted in Fig. 3b. The exponential evolution of thisparameter corroborates the slower polymerization kinet-ics in comparison with conventional bone cements.Moreover, the increase of this parameter could be con-sidered as favorable since it involves the increase of theworking time before the implantation [30].

Finally, it is important to point out that all the modi-fied formulations fulfilled the requirements described inthe ASTM standard for setting time and maximum tem-perature (5—15 min and 90°C).

3.2. Residual monomer content

As it is well known, the presence of residual monomerin the cured mass is due to the restriction in the monomermobility at the end of the polymerization process asa consequence of the gel effect. In this work, the percent-age of monomer present in the total sample was cal-culated by RMN as it had been reported previously [15].In this sense, it is necessary to take into account that inthese modified bone cements the vinylic protons couldarise from the unreacted MMA and from the unreactedTEG.

In Fig. 4, a spectrum of bone cement with 40% (v/v)TEG, where four signals in the vinylic region can beappreciated, is plotted. Therefore, the percentage of resid-ual monomer content was calculated according to thefollowing expression:

% Mr"A6.5A

VA

TB]100 (1)

Fig. 3. Maximum temperature (a) and setting time (b) as a function ofthe percentage (v/v) of TEG in the bone cement formulation.

where AT

is the area of all the protons except thevinylic protons (1—4.5 ppm), A

Vis the area of the vinylic

protons (5.50, 6.20 ppm), and 6.5 is a conversion factordue to the number of protons in the vinyl region and tothe number of the rest of protons of the MMA and TEGunit.

The residual monomer content, expressed as percent-age of unreacted mols related to the initial mols versusthe volume of TEG in the formulation, is plotted inFig. 5. In this figure we can see that this parameterremains practically constant until 40% (v/v) TEGalthough a significant increase can be observed athigher percentages. This behavior is related to theslowing down of polymerization kinetics in for-mulations with high amounts of TEG that leads to higheramounts of unreacted mols. However, a favorablefactor should also be taken into consideration thatcould compensate in some way for the greater residualmonomer content: the lower extractability of this mono-mer from the onset material with respect to that ofthe MMA, due to the greater volume of the monomericunit [31].

456 B. Pascual et al. / Biomaterials 20 (1999) 453—463

Page 5: Modified acrylic bone cement with high amounts of ethoxytriethyleneglycol methacrylate

Fig. 4. Proton NMR spectrum of a cement modified with 40% (v/v) TEG.

3.3. Dimensional changes

3.3.1. Water absorption capacityWhen a glassy polymer is immersed in water, it stead-

ily absorbs water until the process equilibrates. Thewater uptake of methacrylate-based polymers is a diffu-sion-controlled process where the edge effects can beneglected at the early stages [32]. This process followsthe expression:

Mt

M=

"2ADt

nl2B1@2

(2)

where Mtis the mass uptake at time t, M

=the equilib-

rium uptake, l the thickness and D the diffusion coeffi-cient. The later stages up to equilibrium are governed bythe expression

Mt

M=

"1!8

n2

n/=+n/0

1

(2n#1)2expC!

n2D

4l2(2n#1)2tD (3)

The diffusion coefficient, D, can be determined fromEq. (2) by substitution of the uptake measurement. If theuptake, M

t, is measured at suitable intervals of time until

equilibrium is reached, the plot of Mt/M

=against

t1@2 should provide a straight line whose slope is directlyrelated with D in Eq. (2). The value obtained can then besubstituted in Eq. (3) and the calculated values may becompared with the experimental data.

Figure 6 shows the typical plots of the behavior ofwater absorption versus t1@2 for bone cements with 0, 20and 40% (v/v) of hydrophilic monomer in the liquidphase. In all the cements a Fickian diffusion behavior canbe assumed because of the linear dependence at earlyvalues (M

t/M

=(0.5) and the reasonably good agree-

ment of the data with theoretical plots. So, the slope

Fig. 5. Residual mols content as a function of the percentage (v/v) ofTEG in the bone cement formulation.

enables the calculation of the diffusion coefficientsranging the values from 1.6 to 3.6 10~8 cm2 s~1 (Table 1).Diffusion coefficient values are comparable for cementswith lower TEG content and increase noticeably froma percentage of 60% (v/v) TEG. Similar results have beenobtained with hydroxyethyl methacrylate (HEMA)modified bone cements [33]. The increase of diffusioncoefficient of modified bone cement indicates the exist-ence of a higher segmental mobility, or free volumefluctuations, as a consequence of the introduction ofhigher side group monomer. At the later stages the experi-mental absorption curves deviate from the calculated plotssince the absorption process is slower than the theoret-ically predicted due to the concentration dependence.

Another parameter that could be analyzed in theabsorption process was the content of water at the

B. Pascual et al. / Biomaterials 20 (1999) 453—463 457

Page 6: Modified acrylic bone cement with high amounts of ethoxytriethyleneglycol methacrylate

Table 1Values of the diffusion coefficient of bone cements modified with differ-ent proportions of TEG, in water and in saline solution

%TEG D (cm2 s~1) D (cm2 s~1)(v/v) Water Saline solution

0 1.16]10~8 1.62]10~8

10 1.16]10~8 —20 1.54]10~8 0.94]10~8

40 1.39]10~8 0.73]10~8

50 1.64]10~8 1.34]10~8

60 3.38]10~8 1.90]10~8

equilibrium that could be evaluated for each specimenusing the following expression [23]:

Equilibrium gain (%)"

weight of specimen at equilibrium!initial weight

initial weight]100

(4)

The values of equilibrium uptake in water and in salinesolution were plotted against the percentage of TEG inFig. 6. An increase with the enrichment of the cement inthe more hydrophilic component can be seen. However,bone cements with high percentages of TEG reach ap-proximately a constant value. This stabilization could berelated to the weight lose owing to the diffusion of higheramounts of monomer to the solution. On the other hand,the much lower equilibrium uptake in saline solutions isprobably due to the reduction in the osmolarity differ-ence between the external solution and the TEG in thepolymer.

3.3.2. Polymerization shrinkagePolymerization shrinkage associated to setting reac-

tion was determined by measurements of the densityusing the following equation [34]:

% Shrinkage"

Density of polymer!Density of monomer

Density of polymer]100 (5)

This parameter could be affected by the structuralconfiguration as well as the molecular weight of themonomer. In this sense, molecules with lower size lead tohigher percentages of polymerization shrinkage [35].Therefore, the introduction of TEGMA in the bone ce-ment formulation, that has a molecular weight higherthan MMA (2.4 times) and a more voluminous sidegroup, could produce a noticeable decrease in the poly-merization shrinkage. The results collected in Table2 confirm this fact showing lesser polymerization shrink-age as greater percentages of TEG were added to theliquid phase. This feature is of great importance since itcould improve the contact between prosthesis stem and

Fig. 6. Absorption behavior of modified bone cement with differentpercentages (v/v) of hydrophilic component (a) 0% (b) 20% (c) 40%.

Table 2Polymerization shrinkage and porosity for bone cements modified withdifferent proportions of TEG

% TEG (v/v) % Shrinkage % Shrinkage % Porosity(Exp.) (Theo.)

0 4.8 7.72 4.320 4.6 6.91 4.140 4.0 6.01 3.850 3.7 5.66 4.060 3.6 5.24 4.7

bone cement, giving a better transfer of loads through theinterface. To establish data in Table 2, we have taken intoaccount that a part of the cement was already a polymer.In spite of evolution of theoretical and experimental

458 B. Pascual et al. / Biomaterials 20 (1999) 453—463

Page 7: Modified acrylic bone cement with high amounts of ethoxytriethyleneglycol methacrylate

values is similar, noticeable differences between theoret-ical and experimental values can be observed. This re-flects on the one hand the incomplete polymerisationobtained, and on the other the porosity associated tohand mixing, and analysed below.

Another factor which is directly related to the densityand the polymer shrinkage is the porosity of the sample,since cements with reduced porosity contract more dur-ing setting [36]. The determination of polymer densityallows to obtain values of the average percentage ofporosity from the following expression [35, 37]:

% Porosity (%P)"A1!APolymer density

Maximum densityBB]100

(6)

The values of porosity estimated by this indirectmethod can be seen in Table 2 where it can be appreci-ated that all the modified bone cements have practicallythe same percentage of porosity. In all the cases, the dataare in the order of values described in the literature forconventional bone cement formulations [38].

3.4. Mechanical properties

3.4.1. Dynamic mechanical thermal analysisDynamic mechanical analysis (DMTA) is a very valu-

able method to characterize bone cements. Storagemodulus determines inherent rigidity and depends on thematerials’ ability to store mechanical energy. The lossmodulus is associated with the energy absorbed by ma-terial to increase its segmental molecular vibration ortransition of chain positions during dynamic deforma-tion. The dynamic loss modulus, or damping (tan d) is theratio between the loss modulus and the storage modulus.It is sensitive not only to many kinds of molecularmotion, but also to various transitions, relaxation pro-cesses, structural heterogenities, and to the morphologyof multiphase systems. Therefore, interpretations of thedynamic mechanical properties at the molecular level areof great scientific and practical importance in under-standing the mechanical behavior of polymers.

In the region where the dynamic-modulus-temper-ature curve has an inflection point, the internal friction(tand) curve goes through a maximum. This dispersionoccurs in the glass transition region. In this region thedamping is high owing to the initiation of micro-Brownian motion in molecular chain. Some of the mo-lecular chain segments are free to move, while others arenot. Micro-Brownian motion is concerned with thecooperative diffusional motion of the main chain seg-ments. The transition is so conspicuous that it is calledthe primary dispersion (the a-peak).

Therefore, it is easy to understand that the influence ofthe addition of a plasticizer must be noticeable. In thiscase this effect will be done by the absorbed water. So,

with the aim of evaluating the influence of the hy-drophilic character of TEG in the modified bone cementsthe effect of water in the viscoelastic behavior was ana-lyzed. The analysis of DMTA was carried out by meansof testing dry and wet samples after reaching the equilib-rium in physiologic conditions.

3.4.1.1. Dry samples. The results obtained from the testsof dry samples with different percentages of TEG areplotted in Fig. 8 for storage modulus (E@) and tan d. Thespectra showed the classical behavior of the interpenet-rating networks.

As expected, the modulus and the softening point ap-pear at lower temperatures as the TEG content increases(Fig. 8a). This behavior is related to the lower ¹

'of

PTEG with respect to that of the PMMA. This featuregives rise to the formation of less rigid copolymers, that isexperimentally reflected in a lesser value of the modulus.This is a result of the chemical structure of the newmonomer which because of its longer side chain giveshigher mobility to the main chain segments.

The loss tangent curves versus temperature (Fig. 8b)are not symmetrical, being broader at high proportionsof hydrophilic monomer showing at least two peaks. Thepeak at approximately 100°C of unmodified cement os-cillated around this temperature in all the samples pre-pared with different amounts of TEG, so that it is relatedto the relaxation of the PMMA beads. The asymmetrybecomes clearer as the TEG content increases. This be-havior can be related to the presence of different phasesin the matrix structure that could be: PMMA beads thatabsorb MMA monomer preferably, the copolymer withTEG polymerized from this absorbed monomer and thecopolymer TEG/MMA of the matrix. This heterogeneityof chemical composition has become more noticeablebecause of the tendency of one of the monomers to bemore reactive (MMA) than the other (TEG) [39].

Fig. 7. Equilibrium water uptake as a function of the percentage ofTEG in the bone cement formulation.

B. Pascual et al. / Biomaterials 20 (1999) 453—463 459

Page 8: Modified acrylic bone cement with high amounts of ethoxytriethyleneglycol methacrylate

Fig. 8. Tangent delta and dynamic storage modulus versus temper-ature for dry bone cements modified with different percentages (v/v) ofTEG: (d) 0%; (s) 10%; (]) 30%; (n) 50%; (h) 60%.

3.4.1.2. Wet samples. The results of the tests of wet sam-ples are plotted in Fig. 9. The storage modulus presentsa similar behavior to the dry state until the formulationwith a 40% (v/v) TEG. However, the addition of higherpercentages of TEG produces an appreciable change inthis parameter obtaining lower values and a drastic de-crease of this parameter with the addition of 60% (v/v) ofTEG.

In Fig. 9b are plotted the results obtained for deltatangent. From a 20% (v/v) of TEG two peaks can beobserved, that could be related, newly, to structural het-erogenities of the cement. The clearer separation of thepeaks showed in wet samples, is attributed to the ingressof water into the cements. Theoretically, the presence ofcomponents of low molecular weight (plasticizer) in thestructure of polymers makes it easier for changes inmolecular conformation to occur, and so, the temper-ature of the glass transition will be lower. These compo-nents also broaden the loss peak [41].

The influence of the water absorbed in tan d is reflectedin the displacement of the peak attributed to thecopolymer MMA/TEG to low temperatures. This point

out that the hydrophilic component is more sensitive tothe plastificant effect.

3.4.2. Static propertiesTaking into account the influence of water in the

results obtained in the DMTA test, prior to testing, thespecimens for compressive and tensile tests were immer-sed in saline solution at 37°C, in order to approach thebody conditions.

The results of compressive test as compressive strengthand compression modulus are collected in Table 3. Thevalues in both cases show an important decrease with theTEG enrichment of the modified bone cement. Thisbehavior, as in DMTA test, could be attributed to theincrease of the chain mobility as a consequence ofthe longer side group of TEG, and to the increase of theplastification effect.

The characteristic strain—stress curves of tensile test forTEG modified bone cement can be seen in Fig. 10a. Itclearly marked the modification in the mechanical be-havior with the introduction of the hydrophilic mono-mer. Conventional bone cement has a brittle behaviorwith low fracture strain. However, the addition of TEGproduces an important increase of strain to failure, to-gether with a slight decrease in the Young’s modulus,that could be related to a more ductile behavior. This factcan be attributed to the plasticizing effect of water ab-sorbed by the cement, together with the structuralchanges introduced by a monomer with a longer sidechain that contributes to the higher deformation capacityof the system. Similar results have been obtained inHEMA modified bone cements, but with a more drasticdecrease when a 20% of HEMA was added to the formu-lation [34]. Figure 10b shows clearly an increase of thetensile toughness when cements are formulated witha content up to 50% TEG.

The tensile parameters obtained from these curves,Young’s modulus, strain to failure and ultimate stress,

Table 3Mechanical parameters obtained from the compression test of wetsamples for bone cements modified with different proportions of TEG

% TEG (v/v) pc (MPa) Ec (MPa)

0 90.8 1872(9.6) (183)

10 81.5 1726(1.6) (116)

20 72.7 1762(1.6) (132)

40 62.6 840(2.0) (62)

50 46.3 576(10.2) (52)

Note: The standard deviation is given in brackets.

460 B. Pascual et al. / Biomaterials 20 (1999) 453—463

Page 9: Modified acrylic bone cement with high amounts of ethoxytriethyleneglycol methacrylate

Fig. 9. Tangent delta and dynamic storage modulus versus temper-ature for wet bone cements modified with different percentages (v/v) ofTEG: (d) 0%; (s) 10%; (]) 30%; (n) 50%; (h) 60%.

are plotted in Fig. 11. The decrease of the modulus withhigher percentages of TEG could be fitted to a straightline (r"0.973) with only a low decrease from 1.41 to 1.08GPa when a 60% (v/v) TEG is added. Maximumstrength (Fig. 11b) decreases linearly with the addition ofhigher amounts of hydrophilic component (r"0.984)maintaining acceptable values for modified bone cementswith a 20—30% (v/v) TEG. The evolution of strain tofailure is plotted in Fig. 11c where an appreciable in-crease can be seen from 2.8 in conventional formulationto 5.5 in 50% (v/v) TEG modified bone cement. It isimportant to point out that the increase in strain isalmost of 100% with respect to the conventional formu-lations. These cements have a lower modulus of elasticityand greater ductility, that was one of the objectives of thisstudy. This fact has a dual significance: first, a lowermodulus will reduce the applied stress to which it issubjected the prosthesis and second the greater ductilitymeans that a higher strain is required to produce cracking.

The SEM fractographic analysis of fracture surfacesafter the mechanical test of a modified bone cement with20% (v/v) TEG is reported in Fig. 12. Like in the conven-tional cement without TEG, a brittle fracture can beobserved in which PMMA beads can be distinguishedfrom the matrix formed by the copolymerization of the

Fig. 10. (a) Strain—stress curves of bone cements modified with differentpercentages of hydrophilic component. (b) Tensile toughness of bonecements vs %TEG content.

liquid phase. In addition to this, the propagating crackcuts through the beads, that is a reflect of the goodadhesion between phases.

The addition of higher amounts (40% (v/v) TEG) ofhydrophilic monomer gives rise to a slightly differentfracture behavior as it can be seen in Fig. 13. The fracturesurface is not smooth and a higher matrix deformation canbe appreciated, suggesting a more ductile behavior. Theseresults are in accordance with the results of tensile test.

Figure 14 shows the fracture surface of bone cementmodified with a 50% (v/v) TEG. In this fractographya totally different morphology can be observed witha severely deformed matrix in which the PMMA beadscannot be appreciated. A good adhesion between phasescan be appreciated because of the PMMA beads areintegrated perfectly into the matrix formed by the poly-merization of liquid phase.

4. Conclusions

The modification of acrylic bone cements by the re-placement of MMA by about a 30% (v/v) TEG gives rise

B. Pascual et al. / Biomaterials 20 (1999) 453—463 461

Page 10: Modified acrylic bone cement with high amounts of ethoxytriethyleneglycol methacrylate

Fig. 11. Tensile parameters as a function of the percentage (v/v) of TEG(a) Young’s modulus (b) ultimate tensile strength (c) strain to failure.

to an improvement with respect to bone cements basedon PMMA. This formulation presents a reduced temper-ature peak and a longer setting time which could implyan important decrease in bone necrosis and an increaseof the working time before to implantation increase.

The addition of TEG to the bone cement formulationcauses a volume increase of the cement after swelling.These results appear to be interesting at the light of thepossibility of a progressive adjustment of the cement tothe interface bone and prosthesis. In fact, the increase ofvolume could compensate the volume shrinkage occur-ring during cement polymerization, which could producea better tightening of the prosthesis stem in bone cavityimproving, in this sense, mechanical response of the sys-tem. Furthermore, the plastification effect due to the

Fig. 12. SEM photographs of fracture surfaces after tensile test ofa bone cement modified with 20% (v/v) TEG.

Fig. 13. SEM photographs of fracture surfaces after tensile test ofa bone cement modified 40% (v/v) TEG.

Fig. 14. SEM photographs of fracture surfaces after tensile test ofa bone cement modified 50% (v/v) TEG.

462 B. Pascual et al. / Biomaterials 20 (1999) 453—463

Page 11: Modified acrylic bone cement with high amounts of ethoxytriethyleneglycol methacrylate

incoming of water, demonstrated by DMTA, is enhancedand a tougher and more ductile behavior can be obtainedafter swelling.

Acknowledgements

This work was supported by the Comision Interminis-terial de Ciencia y Tecnologıa (CICYT) through theproject MAT 96-0981-C03-02 and Diputacion Foral deGuipuzcoa. The authors also wish to thank the GobiernoVasco for the facilities granted to have this researchperformed.

References

[1] Jasty M, Maloney, WJ, Bragdon CR et al. Histomorphologicalstudies of the long-term skeletal response to well fixed cementedfemoral components, J Bone Jt Surg 1990;72A:1220—9.

[2] Reckling, FW, Dillon WL. The bone-cement interface temper-ature during total joint replacement. J Bone Jt Surg 1977;59A:80—2.

[3] Leeson MC, Lippitt SB. Thermal aspects of the use of polymethylmethacrylate in large metaphyseal deffect in bone. Clin OrthopRel Res 1993;295:239—45.

[4] Liu YK, Stienstra D, Njus GO. The fatigue life of inorgnic bone-PMMA composties. In: Saha S, editor. Biomediak Engineering.I. Recent development. Proc. 1st Southern Biomedical Engineer-ing Conf., New York: Pergamon Press, 1982:12—5.

[5] Wang J-S, Franzen H, Toksvig-Larsen S, Lidgren L. Does vac-uum mixing of bone cement affect heat generation? Analyses offour cement brands. J Appl Biomater 1995;6:105—8.

[6] Jefferiss CD, Lee AJC, Ling RSM. Thermal aspects of self-curingpolymethyl methacrylate. J Bone Jt Surg 1975;57B:511—8.

[7] Meyer P, Lautenschlager EP, Moore BK. On the setting proper-ties of acrylic bone cement. J Bone Jt Surg 1973;55A:149—56.

[8] Kindt-Larsen T, Smith DB, Jensen JS. Innovations in acrylicbone cement and application equipment. J Appl Biomater1995;6:75—83.

[9] Dewijin JR, Driessens FCM, Sloof TJJD. Dimensional behaviourof curing bone cement masses. J Biomed Res Symp 1975;6:99—103.

[10] Crowninshield RD, Brand RA, Johnston RC, Milroy JC. Ananalysis of femoral component stem designed in total hip arthro-plasty. J Bone Jt Srg 1980;62-A:68—78.

[11] Ebramzadeh E, Sarmiento A, Mckellop HA, Linas A, Godan W.The cement mantle in total hip arthroplasty. J Bone Jt Surg1994;76A(1):77—87.

[12] Jasty M, Maloney WJ, Bragdon CR, O’Connor DO, Haire T,Harris WH. The initiation of failure in cemented femoral compo-nents of hip arthroplasties. Br J Bone Jt Surg 1991;73B(4):551.

[13] Mathis RS, Ferracane JL. Properties of glass ionomer resin-com-posite hybrid material. Dent Mater 1989;5:355—8.

[14] Lewis G. Properties of acrylic bone cement. State of the artreview. J Biomed Mater Res 1997;38:155—82.

[15] Pascual B, Gurruchaga M. Gon8 i I, Ginebra MP, Gil FJ, PlanellJA, Levenfeld B, Vazquez B, San Roman J. Mechanical propertiesof a modified acrylic bone cement with etoxyethylenglycol mono-methacrylate. J Mater Sci 1995;6:793—8.

[16] Holm NJ. The relaxation of some acrylic bone cements. ActaOrthop Scand 1980;51:727.

[17] Kusy RP. Characterization of self-curing acrylic bone cements.J Biomed Mater Res 1978;12:271.

[18] Migliaresi C, Fambri L, Kolarik J. Polymerization kinetics, glasstransition temperature and creep of acrylic bone cements. Bio-materials 1994;15(11):875—81.

[19] Vazquez B, Gurruchaga M, Gon8 i I. San Roman J. pH-sensitivehydrogels based on non-ionic acrylic copolymers. Biomaterials1997;18(7):521—6.

[20] Pascual B, Vazquez B, Gurruchaga M, Gon8 i I, Ginebra MP, GilFJ, Planell JA, Levenfeld B, San Roman J. New aspects of theeffect of size and size distribution on the setting parameters andmechanical properties of acrylic bone cements. Biomaterials1996;7(5):507—16.

[21] Sheinin EB, Benson WR, Branon WL. Determination of methylmethacrylate in surgical acrylic cement. J Pharm Sci 1976;65:280.

[22] Rees JS, Jacobsen PH. Dent Mater 1984;5:41.[23] Deb S, Braden M, Bondfield W. Water absorption of modified

hydroxyapatite bone cement. Biomaterials 1995;14:1095—100.[24] Lee AJC. In: Duckworth T, editor. Proc. Symp. on Revision

Arthroplasty. London: Franklin Scientific Publications, 1983:8—13.

[25] Ling RSM. In: Duckworth T, editor. Proc. Symp. on RevisionArthroplasty. London: Franklin Scientific Publications, 1983:14.

[26] ASTM Designation F451-86. Standard specification for acrylicbone cements.

[27] ISO 527-1, 527-2, 1993 (E). Plastic determination of tensile prop-erties (1993). British Standard Institution. B52782: Part 9. Method931A. Preparation and use of multipurpose test specimens, 1988.

[28] Edwards RO, Thomasz FGV. Evaluation of acrylic bone cementand their performance standard. J Biomed Mater Res 1981;15:543.

[29] Shoenfeld CM, Conard GJ, Lautenschlager EP. Monomer releasefrom methacrylate bone cements during simulated in vivo poly-merization. J Biomed Mater Res 1979;13:135—47.

[30] Meyer PR, Lautenschlager EP, Moore BK. On the setting prop-erties of acrylic bone cement. J Biomed Mater Res 1973;55A(1):149—56.

[31] Davy KWM, Braden M. Residual monomer in acrylic polymer.Biomaterials 1991;12:540.

[32] Moore MM, Amtower A, Doerr CL, Brock KH, Dearfield KL.Genotoxicity of acrylic acid, methyl acrylate, ethyl acrylate,methyl methacrylate, and ethyl methacrylate in L5178Y mouselymphoma cells. Environ Mol Mutagen 1988;11:49—63.

[33] Crank J. The mathematics of diffusion. London: Oxford Univer-sity Press, 1956.

[34] Migliaresi C, Capuana P. 2-hydroxymethyl methacrylate modi-fied bone cement. Adv Biomater 1990;9:141—7.

[35] Labella R, Braden M, Davy KWM. Novel acrylic resins for dentalapplications. Biomaterials 1992;13(13):937—43.

[36] Walls AWG, McCabe JF, Murray JJ. The polymerization con-traction of visible-light actived composites resins. J Dent1988;16:177—81.

[37] Flory PJ. Principles of polymer chemistry. New York: CornellUniversity Press, 1953.

[38] Connelly T, Lautenschlager EP, Wixson RL. The role of porosityin the shrinkage of acrylic bone cement. The 13th Annual Meetingof the Society for Biomaterials, 1987:114.

[39] Kusy RP. Characterization of self-curing bone cements. J BiomedMater Res 1978;12:271—305.

[40] Murayama T. Dynamic mechanical analysis of polymeric mate-rials. Materials Science Monographs, Chap. III. Amsterdam:Elsevier, 1982:60—96.

[41] Ward IM, Hadley DW. An introduction to mechanical propertiesof solid polymers, Chap. 9. New York: Wiley, 173.

B. Pascual et al. / Biomaterials 20 (1999) 453—463 463