characterization of new acrylic bone cement based on methyl methacrylate/1-hydroxypropyl...

11
Characterization of New Acrylic Bone Cement Based on Methyl Methacrylate/1-Hydroxypropyl Methacrylate Monomer B. Pascual, I. Gon ˜ i, M. Gurruchaga Departamento Ciencia y Tecnologı ´a de Polı ´meros, Facultad de Quı ´mica, UPV/EHU, San Sebastia ´ n, Spain Received 18 February 1998; accepted 21 August 1998 Abstract: New formulations of acrylic bone cement based on methyl methacrylate/1-hy- droxypropyl methacrylate (MMA/HPMA) monomers were developed with the purpose of obtaining more ductile materials with reduced polymerization shrinkage. In this way, the ductility of such materials increased, but the introduction of high percentages of the hydro- philic component produced an important decrease in Young’s modulus and strength. To ascertain the reason for the deterioration of the tensile parameters, an analysis by scanning electron microscopy of these formulations was carried out; it revealed poor adhesion between the matrix and poly(MMA) beads. We also observed that the polymerization shrinkage increased as the amount of hydrophilic monomer in the formulation decreased, and the 50% (v/v) HPMA modified bone cement compensated for this volume reduction with its water uptake swelling. Measurements taken on the setting time and polymerization exotherm showed a decrease in the former and an increase in the latter, because of the introduction of a more reactive monomer in the bone cement formulation. © 1999 John Wiley & Sons, Inc. J Biomed Mater Res (Appl Biomater) 48: 447– 457, 1999 Keywords: acrylic bone cement; residual monomer; polymerization shrinkage; dynamic mechanical thermal analysis; mechanical properties INTRODUCTION Polymethyl methacrylate (PMMA) is commonly used as a cement for the stabilization of endoprostheses such as the artificial hip and knee, and its introduction in total hip arthro- plasty by Charnley contributed greatly to the success of this procedure. 1 Bone cement fills the space between the bone and the prosthesis to transfer load, and it forms a mechanical bond rather than a chemical bond with the surfaces. 2 Recent studies demonstrated the clinical success of prosthe- ses implanted with cement for up to 20 years of follow-up. 3,4 However, despite this and the numerous improvements of the materials used, loosening remains an impediment to the long- term success of total hip replacements. Thermal necrosis 5 and chemical necrosis due to unreacted monomer release 6,7 were some of the major problems found. In addition to this, the properties at the interfaces are mismatched because the ce- ment is orders of magnitude weaker than the bone or im- plant. 8 Shrinkage of the cement during polymerization 9 may compromise the good load transfer through the interface between the bone and bone cement. Moreover, fatigue and fracture of bone cement were implicated in the failure of these devices. The need to improve some properties of acrylic bone cements is a commonly shared opinion in the literature on the subject. According to most authors, higher dimensional sta- bility, less brittle mechanical behavior, and a more uniform distribution of stress could constitute some of the potential improvements to cement characteristics. Improvement of the mechanical properties of bone cement, specifically, increas- ing resistance to fracture, remains essential for increasing the longevity of cemented total joint arthroplasties. 10 To this end, the development of a cement with lower elasticity modulus, higher ductility, and lower creep resistance than the current formulations is obviously of great interest. By increasing the ductility of the material, the fracture toughness will be in- creased as well; therefore, the material will be less likely to fracture. In addition, it will be more effective at transferring loads from the prosthesis to the bone. To reach this end, a number of formulations have been introduced with constituents that differ marginally or mark- edly from those in the current conventional formulations. The incorporation of beads of polybutyl MA with low glass tem- perature produces a cement with viscoelastic behavior instead of brittle behavior. 11 In the same way, another example is the use of polyethyl MA as a solid component and n-butyl MA (n-BMA) as a liquid monomer. 12 The use of a commercial bone cement, Boneloct, based on methacrylic monomers produced a lower exotherm, lower content of unreacted monomer, and acceptable mechanical properties. 13 PMMA- based copolymer with poly(isobutylene) was used in a bid to Correspondence to: Dr. M. Gurruchaga (e-mail: [email protected]) Contract grant sponsor: Comisio ´n Interministerial de Ciencia y Tecnologı ´a (CICYT); contract grant number: MAT96-0981-C03-02 © 1999 John Wiley & Sons, Inc. CCC 0021-9304/99/040447-11 447

Upload: b-pascual

Post on 06-Jun-2016

217 views

Category:

Documents


4 download

TRANSCRIPT

Characterization of New Acrylic Bone Cement Based on MethylMethacrylate/1-Hydroxypropyl Methacrylate Monomer

B. Pascual, I. Goni, M. Gurruchaga

Departamento Ciencia y Tecnologıa de Polımeros, Facultad de Quımica, UPV/EHU, San Sebastian, Spain

Received 18 February 1998; accepted 21 August 1998

Abstract: New formulations of acrylic bone cement based on methyl methacrylate/1-hy-droxypropyl methacrylate (MMA/HPMA) monomers were developed with the purpose ofobtaining more ductile materials with reduced polymerization shrinkage. In this way, theductility of such materials increased, but the introduction of high percentages of the hydro-philic component produced an important decrease in Young’s modulus and strength. Toascertain the reason for the deterioration of the tensile parameters, an analysis by scanningelectron microscopy of these formulations was carried out; it revealed poor adhesion betweenthe matrix and poly(MMA) beads. We also observed that the polymerization shrinkageincreased as the amount of hydrophilic monomer in the formulation decreased, and the 50%(v/v) HPMA modified bone cement compensated for this volume reduction with its wateruptake swelling. Measurements taken on the setting time and polymerization exothermshowed a decrease in the former and an increase in the latter, because of the introduction ofa more reactive monomer in the bone cement formulation.© 1999 John Wiley & Sons, Inc. J BiomedMater Res (Appl Biomater) 48: 447–457, 1999

Keywords: acrylic bone cement; residual monomer; polymerization shrinkage; dynamicmechanical thermal analysis; mechanical properties

INTRODUCTION

Polymethyl methacrylate (PMMA) is commonly used as acement for the stabilization of endoprostheses such as theartificial hip and knee, and its introduction in total hip arthro-plasty by Charnley contributed greatly to the success of thisprocedure.1 Bone cement fills the space between the bone andthe prosthesis to transfer load, and it forms a mechanical bondrather than a chemical bond with the surfaces.2

Recent studies demonstrated the clinical success of prosthe-ses implanted with cement for up to 20 years of follow-up.3,4

However, despite this and the numerous improvements of thematerials used, loosening remains an impediment to the long-term success of total hip replacements. Thermal necrosis5 andchemical necrosis due to unreacted monomer release6,7 weresome of the major problems found. In addition to this, theproperties at the interfaces are mismatched because the ce-ment is orders of magnitude weaker than the bone or im-plant.8 Shrinkage of the cement during polymerization9 maycompromise the good load transfer through the interfacebetween the bone and bone cement. Moreover, fatigue andfracture of bone cement were implicated in the failure ofthese devices.

The need to improve some properties of acrylic bonecements is a commonly shared opinion in the literature on thesubject. According to most authors, higher dimensional sta-bility, less brittle mechanical behavior, and a more uniformdistribution of stress could constitute some of the potentialimprovements to cement characteristics. Improvement of themechanical properties of bone cement, specifically, increas-ing resistance to fracture, remains essential for increasing thelongevity of cemented total joint arthroplasties.10 To this end,the development of a cement with lower elasticity modulus,higher ductility, and lower creep resistance than the currentformulations is obviously of great interest. By increasing theductility of the material, the fracture toughness will be in-creased as well; therefore, the material will be less likely tofracture. In addition, it will be more effective at transferringloads from the prosthesis to the bone.

To reach this end, a number of formulations have beenintroduced with constituents that differ marginally or mark-edly from those in the current conventional formulations. Theincorporation of beads of polybutyl MA with low glass tem-perature produces a cement with viscoelastic behavior insteadof brittle behavior.11 In the same way, another example is theuse of polyethyl MA as a solid component andn-butyl MA(n-BMA) as a liquid monomer.12 The use of a commercialbone cement, Boneloct, based on methacrylic monomersproduced a lower exotherm, lower content of unreactedmonomer, and acceptable mechanical properties.13 PMMA-based copolymer with poly(isobutylene) was used in a bid to

Correspondence to:Dr. M. Gurruchaga (e-mail: [email protected])Contract grant sponsor: Comisio´n Interministerial de Ciencia y Tecnologı´a

(CICYT); contract grant number: MAT96-0981-C03-02

© 1999 John Wiley & Sons, Inc. CCC 0021-9304/99/040447-11

447

improve the crack and fracture resistance while leaving thestiffness and strength largely unaffected.14 In other studies,biocompatibility and possibilities of using poly(2-hydroxy-ethyl MA) (PHEMA) as a substitute for bone tissues wasexamined.15,16In this context, HEMA modified bone cementspresent tougher and more ductile behavior after swelling inphysiological conditions. All these formulations contributedgreatly to the search for a better bone cement.

Taking into account that the fracture resistance of PMMAcan be enhanced by chemical modification of the liquidphase, the aim of this study was to evaluate new formulationsof acrylic bone cement based on MMA and 1-hydroxypropylmethacrylate (HPMA) as the liquid component. Experienceswith HEMA show that the introduction of a more hydrophilicmonomer in the formulation of conventional bone cementsleads to a less hydrophobic material17 that swells gradually ina physiological environment. After implantation, this swell-ing may compensate for the polymerization shrinkage.18

Moreover, the swelling could improve the mechanical prop-erties as a consequence of the plasticizing effect of water thatcould lead to a more ductile material.19 Because HPMAcopolymerizes easily with MMA and gives a hydrophilicpolymer with known biocompatibility, it was chosen for thisreason in our search for a better bone cement formulation.

As we can see in Figure 1, the chemical structure of bothmonomers differs on the ester side chain, showing the HPMAwith a hydroxyl group in a longer chain. These chemicaldifferences are enough to show differences in polymerizationbehavior, which is an aspect that must be known beforespecimen preparation. A previous study made on HPMA/MMA copolymers synthesized at low polymerization conver-sion, as well as data published in the literature,20 showed ahigher polymerization reactivity of HPMA with respect toMMA. This feature would theoretically give rise to a fasterpolymerization rate. Thus, if this monomer is added to a bonecement formulation, it would be expected to increase thetemperature peak that could affect bone necrosis.5 However,the increase in the polymerization rate is not always a disad-vantage because it would also produce a decrease in thepercentage of the remaining acrylic monomer.21 In additionto this, a previous study22 found that conventional bonecements formulated with greater size beads gave rise to alower temperature peak (;60 °C). Therefore, to introducethis more reactive monomer, HPMA, we chose large PMMAbeads to compensate for an excessive temperature effect.

The effect of the introduction of HPMA on mechanicalproperties was determined experimentally and the surfacefractures were studied by scanning electron microscopy(SEM). Because the water absorption properties of a bonecement are critical to its long-term stabilityin vivo, the effectsof the introduction of HPMA were evaluated under this pointof view. The influence of this component on temperature riseduring polymerization, residual monomer content, water ab-sorption characteristics, and polymerization shrinkage wasalso reported.

MATERIALS AND METHODS

Experimental bone cements were formulated using PMMAbeads, MMA and HPMA monomer, a polymerization initia-tor, and an accelerator. The PMMA beads were ColacrylDP300 (Bonar Polymers Ltd., County Durham, England)with a size distribution of 10 to 150mm, an average diameterof 69.40mm, a number average molecular weight of 2443103, and a polydispersity of 2.87. MMA (Merck) and HPMA(Fluka) stabilized with 100 ppm of monomethylether of hy-droquinone were used as received without further purifica-tion. Benzoyl peroxide (BPO,,1% H2O, Merck) was used asthe initiator after its purification by recrystallization in meth-anol. N,N-Dimethyl-4-toluidine (DMT, Merck) was the ac-celerator of the initiation process and was used as received.

Specimen Preparation

Experimental bone cements based on PMMA were formu-lated by the partial substitution of MMA by HPMA in anamount ranging from 20 to 80% (v/v). In all cases, 1% (v/v)DMT was added to the monomer solution. The solid compo-

Figure 1. Proton NMR spectra of (a) conventional bone cement and(b) modified bone cement with 50% (v/v) HPMA.

448 PASCUAL, GONI, AND GURRUCHAGA

nent consisted of Colacryl DP300 beads to which the corre-sponding amount of finely ground BPO [1.25% (w/w)] wasadded. Although the chemical composition of the liquid com-ponent was changed by the addition of HPMA, the sampleswere obtained by keeping a constant powder/liquid ratio of2/1. All samples were prepared by hand mixing and werecured for 1 h at 37 °C without further treatment beforetesting. Radiopaque agent was not added.

In almost all tests, measurements were carried out using atleast five specimens of each monomeric composition.

Setting Parameters

The temperature and time evolution during the polymeriza-tion reaction was investigated by using a cylindrical Teflonmold, which was described in an earlier work,23 thermostatedat 37 °C. From these studies the most representative curingparameters were obtained: peak temperature and setting time.The peak temperature was considered as the maximum tem-perature reached during the polymerization reaction. Thesetting time was taken as the time when the temperature risewas at a point halfway between the maximum temperatureattained and room temperature. It can be determined accord-ing to the ASTM standard as follows:Troom 1 (Tmax 2Troom)/2, whereTmax is the maximum temperature in degreesCelsius andTroom is the room temperature.

Residual Monomer Content

There are two main reasons to determine and to reducemonomer content: it is a human toxic component and itaffects the physical and mechanical properties of the cement.Moreover, it is known that the use of acrylic bone cementmay cause some death of bone at the implantation site. Thisfact could be related to the polymerization heat5 or to theeffects of the residual monomer.7 It is assumed that a highportion of monomer evaporates from the cement during mix-ing and polymerization, but after hardening some residualmonomer may remain trapped within the polymerized mass ata percentage of about 3%.24

The residual monomer in the cured cement was measuredafter 1 h of polymerization at 37°C by means of1H-NMRspectroscopy.25 The NMR spectra were recorded in a VarianVXR-300 spectrometer operating at 75.5 MHz at room tem-perature. The samples were dissolved using chloroform-d1

and pyridine-d6 as solvents and tetramethylsilane (TMS) asthe internal reference.

The percentage of monomer moles present in the curedcement sample with respect to the total monomer moles used(%Mr) was calculated using the following expression:

%Mr 5 S1.5AV

AMD 3 100,

whereAV andAM are the average area of vinyl and methoxylsignals, respectively, obtained from six specimens and 1.5 is

a factor relating the number of protons in the vinyl region(two) to those of the methoxyl group (three).

Following a similar procedure, the percentage of residualmonomer in the modified cements was evaluated. Figure 1plots the spectrum of an experimental bone cement modifiedwith 50% (v/v) HPMA. Because in this case the vinylicprotons not only arise from the unreacted MMA but also fromthe unreacted HPMA, in this spectrum four signals can beseen in the vinylic region. Therefore, the percentage of re-sidual monomer content in the modified bone cements wascalculated according to the following expression:

%Mr 5 S4 3AV

ATD 3 100.

This equation is a modification of the one used withconventional cements, taking into account the chemical dif-ferences in the structure of the hydrophilic monomer.AT isthe area of all the protons except the vinylic protons (1–4.5ppm), AV is the area of the vinyl protons (5.50, 6.20 ppm),and 4 is a conversion factor that arises from the ratio betweenthe number of protons in the vinyl region and the number ofthe remaining protons of the MMA and HPMA unit.

Dimensional Changes

Water Absorption Capacity. The water sorption kineticsin water and in saline solution were evaluated in two speci-mens of each bone cement maintained in a thermostaticallycontrolled water bath at 376 0.5 °C. The uptake of water wasrecorded at 1-min intervals in the beginning and spacing outthese intervals until the equilibrium was attained. The contentof water at the equilibrium can be evaluated for each speci-men using the following expression26:

equilibrium gain~%!

5weight of specimen at equilibrium2 initial weight

initial weight

3 100.

Polymerization Shrinkage. Polymerization shrinkagewas determined in acrylic resins applied in dentistry by anumber of methods including theoretical calculation,27 a wa-ter filled dilatometer,28 and a density change dilatometer.29

Assuming that almost all shrinkage comes from the poly-merization of the monomer, in this work polymerizationshrinkage associated with the setting reaction was determinedas a function of the density using the following equation30:

% shrinkage5density of polymer2 density of monomer

density of polymer

3 100.

449CHARACTERIZATION OF NEW ACRYLIC BONE CEMENT

A multivolume pycnometer (Micromeritics) apparatus wasused that allows the calculation of absolute density.

Mechanical Properties: Viscoelasticity andStatic Properties

We discuss the assignment of viscoelastic relaxations in amolecular sense to the presence of different chemical groupsin the polymer and in a physical sense to features such as thepresence of different phases. An adequate experimental pro-cedure to determine viscoelasticity is to subject the specimento an alternating strain and simultaneously measure the stressby means of a dynamic mechanical thermal analysis(DMTA). In this work, DMTA was carried out in the bendingmode from 20 to 200 °C by means of a Rheometric ScientificDynamic Mechanical Thermal Analysis MKIII instrument.The specimen (403 10 3 1.5 mm) was tested at a frequencyof 1 Hz with a displacement of 0.064 mm and a heating rateof 3 °C/min.17 The glass transition temperatures (Tg) of thecements were read off as the temperature at which the lossmodulus or loss factor reached a maximum. In order to learnnot only about the material characteristics but also about theeffects of water absorption, dry and wet specimens were usedfor the DMA study.

In clinical service, the implant–cement–bone system issubjected to static or quasistatic direct compressive forcesduring certain activities, such as in a one-legged stance.31

Thus, the quasistatic compressive properties of the bonecement are relevant. Tensile stresses are experienced in var-ious parts of an arthroplasty, for example, on the lateral sideof a hip implant due to bending.32 Thus, the quasistatic tensileproperties of the cement are important.21 In the case at hand,in vitro determination of compressive properties was carriedout in accordance with ASTM F451 in an Instron electrome-chanical testing machine with a crosshead speed of 22 mm/min. Tensile test specimens were prepared according to thestandard specification ISO 527-1 (1993) and BSI B5278sample 1BA. The mechanical tests were carried out at roomtemperature in a servohydraulic MTS Bionix 858 testingmachine with a crosshead speed of 1 mm/min. An extensom-eter was used to measure displacement. The specimens weresoaked until equilibrium was reached in physiological con-ditions (saline solutions at 37 °C) prior to compressive andtensile testing.

SEM

Representative fracture surfaces from the tensile specimenswere analyzed by SEM using a JEOL JSM-6400 microscopeoperating at 20 kV. Each specimen was cut approximately 1cm below the fracture surface and then sputter coated withgold to render the surface electrically conducting.

RESULTS AND DISCUSSION

Temperature and Setting Time

The temperature–time curve during the setting reaction inexperimental bone cements with different amounts of HPMA

are depicted in Figure 2. The influence of the replacement ofMMA by the more hydrophilic MA can clearly be observed.

The evolution of the most representative setting parame-ters of peak temperature and setting time are plotted in Figure3. As expected, the peak temperature increased with theaddition of higher percentages of HPMA to the bone cementformulation as can be seen in Figure 3(a). In Figure 3(b) apractically exponential decrease of the setting time as theamount of HPMA increases can be observed (r 5 0.995,correlation obtained from Kaleida 3.0 for Macintosh). Thisevolution proves that, in fact, polymerization kinetics arefaster than conventional bone cements kinetics. Similar re-sults were obtained by other authors with the addition ofHEMA to bone cement formulations.33

The achieved peak temperature during setting depends onthe total polymerization heat release over a period of time.This feature is related to the amount of polymerizing mono-mer and the chemical composition of the cement compo-nents.34 Thus, as mentioned before, the analysis of HPMA/MMA copolymers synthesized at low polymerization conver-sion is particularly relevant. This study and others reported inthe literature proved the higher reactivity of HPMA withrespect to MMA (rHPMA 5 1.284, rMMA 5 0.261).20 Thisfeature involves an acceleration of the copolymerization re-action that is confirmed by the decrease of the setting time asthe formulation is enriched in HPMA. Therefore, we can saythat the increase of maximum temperature and the decrease ofsetting time are due to the release of heat in shorter periods oftime. In addition, the poor thermal conductivity of the cementmass gives rise to less favorable heat dissipation, leading to ahigher temperature.

Residual Monomer Content

The residual monomer content as a function of the percentageof unreacted moles with respect to the initial moles is plottedin Figure 4 versus the volume of HPMA added to the bonecement formulation. In this figure we can observe a slight

Figure 2. Polymerization exotherm of modified bone cements withvarious percentages of HPMA: (F) 0%, ( ) 20%, (‚), (3) 60%, (E)80%.

450 PASCUAL, GONI, AND GURRUCHAGA

decrease of the unreacted monomer portion down to 40 to50% (v/v) HPMA. If more HPMA is added at this point, anincrease in the percent of residual monomer is produced.

With respect to the increase of toxicity due to the intro-duction of HPMA in the formulation, it would be necessary tomake a further and more in depth study of the biologicalresponse to this material. However, it appears reasonable toassume that the extractability of this monomer from the curedmass would be lower than that of the MMA due to the greatervolume of the monomer unit.35 In this light, the lixiviation tothe body would be reduced. A further study in which thespecimens are stored in saline solution for some time willallow us to confirm this.

Dimensional Changes

The absorption of water by acrylic resins is a phenomenon ofsome importance because it is accompanied by dimensionalchanges and could modify their mechanical behavior. Themagnitude of dimensional changes in bone cements dependson the shrinkage and the swelling characteristics. In this sense

the aim of this work was to obtain a bone cement thatcompensates for the shrinkage with the water uptake at theequilibrium. In Figure 5 the percentages of both dimensionalchanges are plotted.

Because HPMA is a hydrophilic component, bone cementscontaining a higher mole fraction of HPMA are able toaccommodate larger concentrations of water at equilibrium.On the other hand, polymerization shrinkage should be af-fected by the structural configuration, as well as the molec-ular weight, of the monomer. Knowing this, it was found thatsmaller molecules present greater shrinkage.36 Therefore, theintroduction of HPMA in the bone cement formulation,

Figure 4. Residual monomer content as a function of the percentageof HPMA in the bone cement formulation.

Figure 5. Equilibrium water content and polymerization shrinkage asa function of the percentage of HPMA in the bone cement formulation.

Figure 3. (a) Maximum temperature and (b) setting time as a functionof the percentage of HPMA in the bone cement formulation.

451CHARACTERIZATION OF NEW ACRYLIC BONE CEMENT

which has a more voluminous side group, should produce adecrease in the polymerization shrinkage. The results plottedin Figure 5 confirm this, showing less polymerization shrink-age when greater percentages of HPMA are added to theliquid phase.

By comparing both parameters in this figure it can be seenthat at low percentages of HPMA the shrinkage is greaterthan the swelling. However, at high percentages the volumeincrease is greater than the shrinkage and this could producean overstrain in the intramedullary canal. Thus, the optimumvalue is achieved at around 50% (v/v) HPMA, where theshrinkage practically compensates for the swelling. This be-havior is of great importance because it would eliminate thecontact defects between the bone cement and metallic stemand, consequently, could improve the distribution of stress inthe hip prosthesis.37

Viscoelastic Properties

The influence on the viscoelastic behavior of bone cementsby the addition of a more hydrophilic component, HPMA was

analyzed by DMTA by testing dry and wet samples afterreaching the equilibrium uptake in physiologic conditions.

Dry Samples. The mean reading for the storage modulus(E9) and thed tangent (tand) over the temperature range withdifferent percentages of HPMA are depicted in Figure 6. Ingeneral, a similar trend in the evolution of the storage mod-ulus can be observed in all modified bone cements. However,with the introduction of higher percentages of HPMA, aslight decrease of the modulus value across the entire tem-perature range can be seen. This behavior is related to thelower Tg of PHPMA (72 °C)38 with respect to that of thePMMA, which gives rise to the formation of less rigid co-polymers. This is reflected experimentally in a lower value ofthe modulus, although the structural similarity between bothmonomers does not produce noticeable changes.

The curves of thed tangent (tand) over the temperaturerange are plotted in Figure 6(b). The curves are not symmet-rical, and at higher proportions of hydrophilic monomer thepeaks are extended to a wider range of temperatures. Thisbehavior can be related to the presence of different phases inthe cement structure: the copolymer obtained from the liquidcomponent and the PMMA beads of the solid component.

Conventional formulations of bone cement present a rathersharp peak with a maximum at 129 °C, which is associatedwith the glass transition of the system. With the introductionof higher amounts of HPMA, the shape of the tand changesfrom a wider peak with a shoulder that evolves to a secondpeak. This second peak is displaced to lower temperatures asthe percentage of HPMA increases. The peak at higher tem-peratures has only small variations with the addition of dif-ferent amounts of HPMA, and so we believe it could berelated to the relaxation of the solid component. The peak atlower temperatures changes position and shape. This may berelated to the relaxation process of the matrix formed fromthe polymerization reaction of the liquid phase that is HPMAenriched. The values compiled in Table I prove the behaviordescribed.

Wet Samples. The results of the evolution of the storagemodulus and the tand of wet samples are plotted in Figure 7.

Figure 6. Evolution of dynamic storage modulus and tan d withtemperature in dry modified bone cements with different percentagesof HPMA: (F) 0%, (E) 10%, (Œ) 30%, (3) 50%, (h) 60%.

TABLE I. Glass Transition Temperatures Obtained from DMTATest of Dry Samples of Experimental Bone CementsObtained with Various Percentages of HPMA

% HPMA (v/v)

Tg (°C)

First Peak Second Peak

0 — 129.020 106 131.540 104 129.650 102 129.760 106 129.680 99 130.5

100 99 130.2

The values of the first peak are approximates.

452 PASCUAL, GONI, AND GURRUCHAGA

Theoretically, the presence of low molecular weight compo-nents in the polymer structure should displace the transitionsto lower temperatures due to the plasticizing effect.39 Thestorage modulus presents behavior similar to that of the drystate up to formulations with 40% (v/v) HPMA. Nevertheless,the addition of higher percentages of HPMA of approxi-mately 60 to 80% (v/v) produces a noticeable decrease in thisparameter. Figure 7(b) plots the tand over the temperaturerange. In this case, the influence of the water absorbed isreflected in the displacement to lower temperatures of thepeak corresponding to the matrix relaxation. From 20% (v/v)HPMA two peaks can be clearly distinguished that couldagain be related to the solid component and the copolymerformed from the liquid phase.

By comparing the results collected in Table II and Table I,a clear displacement of the matrix relaxation peak at lowertemperatures was observed in wet samples. This is directlyrelated to the plasticizing effect of the water absorbed by thematrix, which is enriched in the hydrophilic monomer.

Mechanical Static Properties

Mechanical properties were evaluated by compressive andtensile tests. Taking into account the influence of water

shown by the DMTA test, prior to testing the specimens werekept in physiological conditions until equilibrium wasreached.

Compressive Test. The results of the compressive testare collected in Table III as a function of compressivestrength and modulus. The strength value is practically con-stant up to 40% (v/v) of the hydrophilic component. Theintroduction of higher percentages produces a decrease that ismore pronounced at 80% (v/v) HPMA. By contrast, thecompressive modulus shows a drastic decrease as the contentof HPMA increases. This drop is related to the plasticizingeffect of the water absorbed, as well as the structure modifi-cation of the matrix by the introduction of HPMA.

Taking into account that the minimum value required bythe ASTM standard for compressive strength is 70 MPa, onlyexperimental bone cement modified with HPMA up to 40%(v/v) could be used as bone cement. However, it is necessaryto point out that the determination of these parameters wasmade for wet samples instead of dry samples and the ASTMstandard referred to tests done in the dry state. This consid-eration is important because the compressive strength valuein the wet state is lower than in the dry state.

Tensile Test. The characteristic curves of the tensile testfor bone cement are shown in Figure 8. We must point outthat we displaced the zero value starting point in order toclearly differentiate between each cement plot; obviously all

Figure 7. Evolution of dynamic storage modulus and tan d withtemperature in wet modified bone cements with different percentagesof HPMA: (F) 0%, (E) 10%, (Œ) 30%, (3) 50%, (h) 60%.

TABLE II. Glass Transition Temperatures Obtained from DMTATest of Wet Samples of Experimental Bone Cements Obtainedwith Various Percentages of HPMA

% HPMA (v/v)

Tg (°C)

First Peak Second Peak

0 114 127.820 91 119.640 71 121.750 68 125.860 63 126.1

100 '30 132.0

The values of the first peak are approximates.

TABLE III. Mechanical Parameters Obtained from CompressiveTest in Wet Samples for Modified Bone Cements with VariousPercentages of HPMA

% HPMA (v/v) sc (MPa) Ec (MPa)

0 74.6 (8.2) 1678 (53)20 81.3 (4.5) 1505 (67)40 82.0 (9.6) 1310 (110)50 51.8 (6.8) 1167 (92)60 47.1 (9.8) 766.9 (92)80 15.6 (1.6) 194.1 (21)

The standard deviations are given in parentheses. Five specimens were tested foreach material.

453CHARACTERIZATION OF NEW ACRYLIC BONE CEMENT

of them start from a zero value strain at zero stress. Theinfluence on the mechanical behavior of adding hydrophilicmonomer can be clearly observed. Furthermore, conventionalbone cement has brittle behavior with low fracture strain.However, the addition of HPMA changes the mechanicalbehavior of these cements by slightly increasing the fracturestrain and maintaining at least the toughness up to 20%HPMA content formulations.

The tensile parameters obtained from these curves(Young’s modulus, strain to failure, and ultimate stress) areplotted in Figure 9. The decrease of the modulus with higherpercentages of HPMA fits to an exponential behavior (r 50.973), and there was a decrease to one-third of the originalvalue when 80% (v/v) HPMA was added. The maximumstrength [Fig. 9(b)] decreases exponentially (r 5 0.984) withthe addition of higher amounts of hydrophilic component.However, this parameter maintains acceptable values formodified bone cements with 20 to 30% (v/v) HPMA. Theevolution of strain to failure in Figure 9(c) shows a consid-erable increase from 3.4% of the conventional formulation to7.2% of the 50% (v/v) HPMA bone cement. It is important topoint out that the increase in strain is practically a 100%increase with respect to conventional formulations. This canbe attributed to the water absorption that enlarges the freevolume between the macromolecules, allowing greater defor-mation of the material before fracture.

The decrease in tensile parameters observed at high con-tents of HPMA could be attributed to poor adhesion betweenthe phases of the cement. This could be related to a deficientwetting of PMMA beads by the liquid component. Theseresults are corroborated by the study of fracture surfacebelow.

SEM

The SEM fractography from the tensile surface of a bonecement modified with 10% (v/v) HPMA is shown in Figure10. Bone cements are considered to be biphasic in nature.1

Charnley described spherical domes that were actually pre-polymerized beads that could be distinguished as incorpo-

rated into a matrix formed by the original liquid component.1

The matrix presents higher deformations than conventionalbone cements because of the introduction of a more hydro-

Figure 9. Tensile parameters as a function of the percentage of HPMA(a) Young’s modulus, (b) ultimate tensile strength, and (c) strain to failure.

Figure 8. Strain–stress curves of modified bone cement with differentpercentages of hydrophilic component.

454 PASCUAL, GONI, AND GURRUCHAGA

philic monomer in the liquid phase. As we mentioned before,this last aspect increases the equilibrium gain and conse-quently the plasticizing effect. However, a brittle fracturebehavior similar to conventional bone cements remains pre-dominant because the propagating crack cuts through thebeads, which is a reflection of the good adhesion betweenboth phases. The addition of higher amounts of hydrophilicmonomer [30% (v/v) HPMA] offers the same behavior(Fig. 11).

Figure 12 shows the surface fracture of a modified bonecement with 50% (v/v) HPMA. In this photograph a totallydifferent morphology can be observed. The adhesion betweenthe PMMA beads and the matrix is not good enough, becausethe propagating crack goes around the bead along the bead–matrix interface. In this fractography the PMMA beads de-tached from the matrix and the holes in which they were

located before testing can be observed. These results are inaccordance with the evolution of tensile parameters.

This behavior is shown more clearly in 80% (v/v) HPMAmodified bone cements (Fig. 13). In this case, the cementpresents insufficient wetting of PMMA beads together withpoor adhesion that gives rise to a removal of the beads fromthe matrix. This photograph, in contrast with Figure 12,clearly shows beads detached completely from the matrix,indicating a deterioration of the adhesion by the addition ofhigher percentages of HPMA. The fractographic observationclearly shows the resulting deteriorated adhesion betweenboth phases in modified bone cements with high percentagesof HPMA.

CONCLUSIONS

In general, bone cement formulations modified with HPMAshow some improvements with respect to conventional for-

Figure 10. SEM fractograph of the fracture surface after the mechan-ical test of a modified bone cement with 20% (v/v) MMA substitutedby HPMA.

Figure 11. SEM fractograph of the fracture surface after the mechan-ical test of a modified bone cement with 30% (v/v) MMA substitutedby HPMA.

Figure 12. SEM fractograph of the fracture surface after mechanicaltest of a modified bone cement with 50% (v/v) MMA substituted byHPMA.

Figure 13. SEM fractograph of the fracture surface after the mechan-ical test of a modified bone cement with 80% (v/v) MMA substitutedby HPMA.

455CHARACTERIZATION OF NEW ACRYLIC BONE CEMENT

mulations based on PMMA and present the following note-worthy aspects:

● The percentage of residual monomer content decreaseswith the addition of hydrophilic monomer. This fact alongwith the greater volume of the structural unit decreases thepotential monomer release to the surrounding tissues.Therefore, this feature should be advantageous from theviewpoint of the toxic effect of the cement.

● The addition of greater amounts of hydrophilic componentincreases the water absorption and decreases the polymer-ization shrinkage. In this regard, both parameters could becompensated in formulations with a HPMA content ofabout 50%.

● In spite of an observed decrease in the tensile strength(which is not statistically significant), we can say that theintroduction of HPMA at up to 20% in the cement formu-lation improves the mechanical properties. The worseningof these properties shown by cements formulated withhigher HPMA contents could be related to the poor adhe-sion between the matrix and PMMA beads.

In the light of the results, we can say that the formulationof a bone cement of about 50% HPMA content with enhancedmechanical behavior is a great success of this research.Therefore, a further study toward the optimization of theadhesion between phases to improve mechanical propertieswill be of great interest.

Financial support was provided by the Comisio´n Intermisinisterial deCiencia y Tecnologı´a (CICYT) through the project MAT96-0981-C03-02.The authors also thank the Exma Diputacio´n de Guipu´zcoa for the facilitiesin which this research was performed.

REFERENCES

1. Charnley J. Fracture of femoral prostheses in total hip replace-ment, Clin Orthop Rel Res 1975;111:105–120.

2. Ahmed AM, Raab S, Miller JE. Metal/cement interface strengthin cemented stem fixation. J Orthop Res 1984;2:105–118.

3. Schulte KR, Callaghan JJ, Kelley SS, Johnston RC. The out-come of Charnley total hip arthroplasty with cement after aminimum twenty-year follow-up. J Bone Joint Surg 1993;75A:961–975.

4. Kavanagh BK, Wallrichs S, Dewitz M, et al. Charnley low-fracture arthroplasty of the hip: twenty-year results with cement.J Arthroplasty 1994;9:229–234.

5. Dipisa JA, Sih FS, Berman AT. The temperature problem at thebone–acrylic cement interface of the total hip replacement. ClinOrthop Rel Res 1979;121:95–98.

6. Feith R. Side effect of acrylic cement implanted into bone. ActaOrthop Scand Suppl 1975;161:1–36.

7. Petty W. Methyl methacrylate concentrations in tissues adjacentto bone cement. J Biomed Mater Res 1980;14:88–95.

8. Liu YK, Stienstra D, Njus GO. The fatigue life of inorganicbone–PMMA composites. In: El Saha S, editor. BiomedicalEngineering, Proceedings of the First Southern Biomedical En-gineering Conference. New York: Pergamon Press; 1982. p12–15.

9. Dewijin JR, Driessens FCM, Sloof RJJD. Dimensional behav-iour of curing bone cement masses. J Biomed Mater Res Symp1975;6:99–103.

10. Topolesky LDT, Ducheyne P, Cuckler JM. A fractographicanalysis of in vivo poly(methyl methacrylate) bone cementfailure mechanisms. J Biomed Mater Res 1990;24:135–154.

11. Litsky AS, Rose RM, Robin CT, Trasher EL. A reduced-modulus acrylic bone cement: preliminary results. J Orthop Res1990;8:623–626.

12. Weightman B, Freeman MAR, Revel PA, Braden M. Mechan-ical properties of cement and loosening of the femoral compo-nent of hip replacement. J Bone Joint Surg Br 1987;69:558–564.

13. Kindt–larsen T, Smith DB, Steen J. Innovation in acrylic bonecement and application equipment. J Appl Biomater 1995;6:75–83.

14. Asken MJ, Keszler B, Greenr K, Richard GC, Kenedy JP.Toughness and crack propagation rates of bone cement and apolyisobutylene-toughened polymethyl methacrylate material.In: Proceedings of the 20th Annual Meeting of the Society forBiomaterials, Boston. 1994. Minneapolis, MN: Society for Bio-materials. p 417–420.

15. Korbelar P, Vacik J, Dyleusky I. Experimental implantation ofhydrogel into bone. J Biomed Mater Res 1988;22:751–762.

16. Stmentana K Jr, Stool M, Kiorbelar D, Novak M, Adam M.Implantation of p(HEMA)–collagen composite into bone. Bio-materials 1992;13:639–642.

17. Migliaresi C, Fambri L, Kolarik J. Polymerization kinetics,glass transition temperature and creep of acrylic bone cements.Biomaterials 1994;15:875–881.

18. Patel MP, Braden M, Davy KMW. Polymerization shrinkage ofmethacrylate esters. Biomaterials 1987;8:53–56.

19. Shen J, Chen CC, Sauer JA. Fatigue of PMMA: effects ofmolecular water content and frequency. In: Fatigue in PolymersConference Proceedings. London: Edited by The Plastic andRubber Institute; 1983.

20. Gaddam NB, Xavioir SF, Goel TC. Copolymerization of 2-hy-droxypropyl methacrylate with alkyl acrylate monomers. JPolym Sci Polym Chem Ed 1977;15:1473–1478.

21. Lewis G. The properties of acrylic bone cement: a state-of-the-art review. J Biomed Mater Res (Appl Biomater) 1997;38:155–182.

22. Pascual B, Vazquez B, Gurruchaga M, et al. New aspects of theeffect of size and size distribution on the setting parameters andmechanical properties of acrylic bone cements. Biomaterials1996;7:507–516.

23. Petty W. Methyl methacrylate concentration in tissues adjacentto bone cement. J Biomed Mater Res 1980;14:88–95.

24. Schoenfeld CM, Conard GJ, Lautenschlager EP. Monomer re-lease from methylmethacrylate bone cement during simulated invivo polymerization. J Biomed Mater Res 1979;13:135–147.

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

26. Deb S, Braden M, Bonfield W. Water absorption of modifiedhydroxyapatite bone cement. Biomaterials 1995;14:1095–1100.

27. Coleman MM, Graft JF, Painte PC. Specific interactions and themiscibility of polymer blends. Lancaster, PA: Technomic Pub-lishing; 1991.

28. Hay JN, Shortall AC. Polymerization contraction and reactionkinetics of three chemically activated restorative resins. J Dent1980;16:172–176.

29. Ress JR, Jacobsen DE. The polymerization shrinkage of com-posite resins. Dent Mater 1989;5:41.

30. Labella R, Braden M, Davy KWM. Novel acrylic resins fordental applications. Biomaterials 1992;13:937–943.

31. Lee AJC. In: Proceedings of the Symposium on Revision Ar-throplasty. Duckworth T, editor. London: Franklin ScientificPublications; 1983. p 8–13.

456 PASCUAL, GONI, AND GURRUCHAGA

32. Ling RSM. In: Proceedings of the Symposium on RevisionArthroplasty. Duckworth T, editor. London: Franklin ScientificPublications; 1983. p 14.

33. Yang JM. Polymerization of acrylic bone cement using differ-ential scanning calorimetry. Biomaterials 1997;18:1293–1298.

34. Efferis CDJ, Lee JC, Ling RSM. Thermal aspects of self-curingpolymethylmethacrylate. J Bone Joint Surg 1975;37B:511–518.

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

36. Walls AWG, McCabe JF, Murray JJ. The polymerization con-traction of visible-light actived composites resins. J Dent 1988;

16:177–181.37. Harrigan TP, O’Connor JA, Burke DW, Harris WH. A finite

element study of the initiation of failure of fixation in ce-mented femoral total hip component. J Orthop Res 1992;10:134 –144.

38. Shen MC, Srtong JD, Matusik FJ. The effect of the hydrogenbonds on the dynamic mechanical properties of glassy poly-methacrylates from 77°C. J Macromol Sci (Phys) 1967;B1:15–27.

39. Ward IM, Hadley DW. An introduction to mechanical proper-ties of solid polymers. London: Wiley; 1993. p 173.

457CHARACTERIZATION OF NEW ACRYLIC BONE CEMENT