Combined Effect of Polishing on Surface Morphology and Elastic Properties of a Commercial Dental Restorative Resin Composite

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Science of Advanced MaterialsVol 4 pp 126ndash134 2012(wwwaspbscomsam)

Combined Effect of Polishing on SurfaceMorphology and Elastic Properties of aCommercial Dental Restorative Resin CompositeMarco Salerno1lowast Niranjan Patra1 2 Sanjay Thorat1 2 Giacomo Derchi3 and Alberto Diaspro1

1Italian Institute of Technology via Morego 30 I-16163 Bolzaneto Genova Italy2University of Genova viale Causa 13 I-16145 Genova Italy3Tirrenian Stomatologic Institute via Aurelia 335 I-55041 Lido Di Camaiore Lucca Italy

ABSTRACT

We have investigated the effect of polishing with two different systems namely Venus Supra and Enhance onthe properties of a resin composite currently in use for dental restorations namely Venus Diamond On boththe non-polished and polished specimens first the material surfaces have been imaged by scanning electronmicroscopy and atomic force microscopy and quantitative information on the surface roughness has beenextracted Then the elastic properties of the composite have been measured in compression by instrumentedindentation The measured reduced modulus and hardness of the non-polished material were (127plusmn20) GPaand (435plusmn 105) MPa respectively The polishing affected the elastic properties only in the case of VenusSupra system which apparently decreased both modulus and hardness of sim10 and sim20 respectivelywhen measured at a maximum indentation depth of sim2 m However at higher indentation depth of sim5 mthis difference disappears We tentatively assign the apparent decrease observed in the elastic properties tothe material smear distributed on the surface by the polishing

KEYWORDS Dental Composites Polishing Roughness Elastic Modulus AFM SEM

1 INTRODUCTION

In all the fields of science and technology the relationshipbetween material patterning and its properties is a criticalissue for the desired functionality of a device The rela-tionship between micro-nanoscale features and their func-tion has been clear since decades in electronics1 wherean extensive use of traditional radiation based lithographyhas been made2 and has more recently reached the fieldof photonics first34 and mechanics later on5 where themodern soft lithographies are preferentially used instead6

In the field of biomaterials an additional concern for thesurface texture is that of the interaction with the respec-tive biological systems For dental materials for exam-ple the effect of surface roughness on bacterial adhesionand biofilm formation finally giving rise to dental plaquehas already been clearly pointed out7 In recent years alsodental materials science and technology is undergoing anincreasing application of nanoscience and nanotechnologyconcepts and analytical methods89 In this streamlinewe have tried to explore the possible connections between

lowastAuthor to whom correspondence should be addressedEmail marcosalernoiititReceived 15 December 2010Accepted 30 March 2011

surface morphology and functionality of a dental restora-tive material on a submicrometric scale by means ofadvanced techniques such as atomic force microscopy(AFM) and instrumented indentation called microindenta-tion in the followingIn this work we have selected one dental resin com-

posite of common use which we had already investigatedrecently for its response to air-polishing10 Specimens ofthis composite have been treated with milling such as forshaping the material surface in an actual dental restorationin vivo and polishing has been subsequently performedon the milled surfaces with two different systems Twodifferent control samples (negative and positive control)have also been considered as the reference for our mea-surements as described in Section 212 The effect ofthe different surface treatments on the surface morphol-ogy have been first characterized by means of both scan-ning electron microscopy (SEM) and AFM which allowedus to measure the surface roughness Then the elasticmechanical properties have been investigated by means ofmicroindentationStudies of the effect of polishing on the surface of

dental restorative composites are common however theinvestigation is generally restricted to the morphologi-cal effects only11ndash14 On the contrary in this work we

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Salerno et al Effect of Polishing on Roughness and Modulus of a Dental CompositeARTIC

LEfocused our attention mainly on the elastic mechani-cal properties of the polished composite Additionallythe experimental analysis of the sample surfaces hasbeen complemented with SEM microanalysis via energy-dispersive X-ray spectroscopy (EDS)

2 MATERIALS AND METHODS

21 Sample Preparation

The resin composite of choice is Venus Diamond (HeraeusKulzer Duumlbendorf Germany) shortly VD in the follow-ing VD is a dental restorative material available on theEuropean market since approximately two years which isinnovative in both organic matrix formulation and fillerparticles15 According to a recent filler size-type rankingof resin dental composites VD can be termed as a lsquomid-ifillrsquo in that it contains both nanofillers and microfillersin a diameter range of 5 nmndash20 m1617 The filler parti-cles are made of Barium Aluminium Fluoride glass andthe nominal filler loading is 64 by volume16 The interestof our group has recently focused on VD as an exemplarymaterial for universal restorations ie for both anteriorand posterior teeth of which we aim to deepen our cur-rent understanding of the basic properties in order to beable to associate them with correct clinical performanceexpectations10

211 Specimen Fabrication

Square specimens of VD (sim10times10 mm2 sim15 mm thick-ness) were prepared by compressing the composite resinpaste into a polyethylene hollow matrix with an amalgamcondenser and removing the excess material The speci-mens were then irradiated through a transparent acrylatestripe for 40 s with a blue (440ndash480 nm wavelength) LEDcuring lamp Translux Power Blue (Heraeus Kulzer) withan irradiance of sim1 Wcm2 16 specimens of VD wereprepared that were subdivided into 4 groups dedicated todifferent surface treatments namely the two polishing sys-tems and a negative and a positive control

212 Specimen Polishing

Two different polishing systems were tested namelyVenus Supra (Heraeus Kulzer) and Enhance (DentsplyCaulk DE USA) shortly VS and EN in the followingThese polishing systems were selected after their best over-all performance (ie lowest resulting surface roughness) ina group of three polishing systems investigated in a pre-vious study14 VS is a one-step polishing system basedon silicone-impregnated polishing points EN is a multi-step polishing system combining pointed shape polishingpoints with a polishing cup with abrasive paste with overallalumina abrading particles in the range of 03ndash1 m diame-ter As a negative control a non-polished non-milled sample

has been used Additionally as a positive control a milledbut non-polished sample has been considered These sam-ples are called lsquonegativersquo and lsquopositiversquo control since theirresulting roughness is expected to be lesser or larger thanthat of the polished samples respectivelyThe polishing procedure was performed always by the

same expert operator according to the manufacturerrsquosinstructions with a process time of 20 s to reproduce theclinical practice After polishing the samples were blownwith air and stored in air at room temperature for one weekbefore the analysis For each sample four specimens wereprepared and analyzed

22 SEM Analysis

SEM measurements were carried out at 15 kV electronbeam acceleration voltage with a JSM-6490LA (JEOLLtd Tokyo Japan) collecting secondary electron emissionimages of the composite sample surface at both 2000 and10000 magnification (X) The samples did not require anymetal or carbon coating thanks to the low vacuum operat-ing conditions (25times10minus1 mbar) In some cases after SEMimaging EDS was also performed for identification of theabundance of elemental atomic species on the material sur-face on smaller selected areas of interest

23 AFM Analysis

The measurements have been taken with a commercialMFP-3D AFM (Asylum Research Santa Barbara USA)The relative height maps have been acquired in air intapping mode with silicon probes NSG10 (NT-MDTZelenograd Russia) which had nominal spring constantand fundamental resonance frequency values of sim10 Nmand sim250 kHz respectively The roughness of each spec-imen has been evaluated as the root mean square value(RMS) of the distribution of heights in the AFM topo-graphical images Optimum scan area was estimated to be30times 30 m2 (ie 30 m scan size) chosen such as tobe representative of a region typically affected by attach-ment of several oral bacteria adhering to the dental com-posite surface when performing in vivo18 For each sample4 specimens were measured in 3 different regions each(n= 12)

24 Microindentation

Microindentation experiments were performed by usinga NanoTest setup (Micro Materials Ltd Wrexham UK)equipped with a Berkovich three-sided pyramid diamondindenter with nominal radius of curvature at the tip ofsim50 nm The indentations were performed in a cabi-net with constant temperature of 23 C in mechanicaland electrical low-noise conditions under load-controlThe setup was first calibrated by five iterative indentationcycles into fused silica (with maximum load increasing

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Effect of Polishing on Roughness and Modulus of a Dental Composite Salerno et alARTIC

LEfrom sim05 to 200 mN in 40 mN steps) In a subsequentphase the samples were indented with the same experi-ment program (see Fig 4) For each specimen indenta-tions were repeated four times on different regions witha 001 mNs loading and unloading rate and a 60 s dwellperiod at maximum load

25 Statistical Analysis

The differences between the four groups of surface treat-ments were analyzed for statistical significance by meansof ANOVA procedure with Bonferronirsquos post-hoc testusing Origin 80 software (Originlab Northampton USA)Various levels of significance (typically p lt 001 andp lt 005) have been tested for statistical relevance of thedifference

3 RESULTS AND DISCUSSION

31 Surface Morphology

In Figures 1 and 2 representative SEM and AFM imagesof the different samples are shown respectively It can beobserved that both imaging techniques have their advan-tages and disadvantages SEM imaging is faster than AFM(time per frame of the order of sim10 sec vs sim10 minrespectively) and allows for a larger view of the samplesurface (up to mm scale scan size instead of maximum90 m value of AFM) This latter point combined withthe relatively large size (5ndash10 m diameter) of some filler

Fig 1 Representative SEM images of the considered samples (a) negative and (b) positive controls and samples polished with (c) VS and (d) ENMagnification 2000times

particles appearing in Figure 1 allows one for a betteroverview of the specimen surface by using SEM insteadof AFM Also in the SEM images a clear contrast isobserved between the above mentioned large fillers andthe surrounding area mainly due to the high local smooth-ness and peculiar surface orientation of the individualparticlesIn the AFM images (Fig 2) on the contrary a clear

distinction of filler particle edges is rarely obtained on therough surfaces due to both the larger probe size of thesilicon pyramid tip as compared to the focused electronbeam and to the different type of probe-sample interac-tion In fact in AFM imaging relies on a real contact andforce sensing also resulting in artifacts such as strikes andprobe tip motion defects occasionally due to the presenceof contaminants adsorbed on the surface in air (see thefeatures enclosed in the red lines in Fig 2) On the otherhand one important advantage of AFM is that it allows fora real-space 3D measurement of the surface topographyAs a consequence direct evaluation of the RMS roughnessis possible which is of interest for a dental material com-posite for both examination of its wear and for evaluationof the possible correlation with bacterial biofilms formingon its surface

32 Surface Roughness

Already in the SEM images (Fig 1) it clearly appearsthat the negative control (Fig 1(a)) is much smoother than

128 Sci Adv Mater 4 126ndash134 2012


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LEFig 2 Representative AFM images of the considered samples (a) negative and (b) positive controls and samples polished with (c) VS and (d) ENScan size 30 m vertical scales (fitting to mean of height distribution plusmn1 standard deviation are 250 1500 1100 and 1400 nm respectivelyMasked regions correspond to heights above +2 from mean

all the other samples where scratches (Fig 1(b)) hol-lows (Fig 1(d)) and larger filler particles (Figs 1(bndashd))appear In particular since the cured composite material isexpected to be uniform in volume distribution of the fillerparticles the absence of the large fillers in the negativecontrol (Fig 1(a)) can be associated with their sinking induring the curing leaving a top surface covered with poly-mer matrix and smaller filler particles only This top layervoid of large fillers is obviously removed by milling asobserved in the positive control (Fig 1(b)) Unfortunatelydespite the qualitative appearance of increased roughnessSEM does not allow for its quantitative measurement in adirect mannerOn the other hand in the AFM images of Figure 2 a

color shade is directly associated with the relative sam-ple features height as described in the vertical side-barsFor example for the scratches visible in the SEM imageFigure 1(b) the step-height can also be evaluated in theAFM image Figure 2(b) as coded by the z values There-fore from the AFM images we could extract the rough-ness data Before determining the RMS of the heights

the probe-sample contact defects appearing in the AFMimages due to adsorbed contaminants have been excludedby thresholding the respective images for z le mean value+2 standard deviation (see red line edges in Figs 2(andashc))and calculating only on the masked areas The rough-ness data have been summarized in Figure 3 where theRMS means plusmn1 standard deviation have been plotted(n= 12)As expected the negative control provides the smoothest

surface among all the samples Furthermore the positivecontrol is on the contrary the roughest sample of allIn fact polishing with either of the two systems decreasedthe surface roughness with respect to the positive controlsuch that the polished samples display intermediate valuesof RMS in between the two above opposite control limitsHowever both the polished samples are much closer to thepositive control than to the negative oneConcerning in particular the outcome of the two pol-

ishing systems these are consistent with previous resultsobtained on specimens of the same material prepared inthe same way yet measured with a different AFM by



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LE

Fig 3 RMS roughness of the samples (means plusmn1 after different sur-face treatment The line joins statistically different groups (lowastlowastp lt 0001)The other pairs of groups are not significantly different (p gt 005)

a different operator and with different probes14 Indeedbetween VS and EN there is only a slight difference+16 in RMS for EN in our case as compared tothe respective uncertainty (standard deviation above meanvalue) of 40 on the RMS values obtained for both meth-ods The absolute numerical values are also in reasonableagreement with Ref [14] In fact if one considers that thesurface area investigated here is smaller that in that case(30times 30 m2 instead of 50times 50 m2 the correspond-ing RMS values are also decreased (between sim280 nmand sim310 nm in the present case with respect to betweensim600 nm and sim620 nm of Ref [14]) in a sublinear pro-portion (RMS decrease down to sim12 for a decrease inarea own to sim13)In the mentioned reference work14 only a negative con-

trol was considered and the absolute RMS values of thepolished samples were quite close to it (+16 for themean of the polished samples) In our study on the con-trary the polished samples are much rougher (+86 inRMS) than the negative control and on the contrary quiteclose to the positive control of approximately the sameamount of the referenced work yet with different direction(minus19 in RMS) Obviously whatever the reference con-trol sample both polishing treatments applied for the con-sidered times are only affecting the surface roughness toa limited extent (approximately plusmn20 maximum changein RMS)The effective significance of the different RMS values

in Figure 3 has been analyzed with descriptive statisticsvia an ANOVA test It turned out that the negative con-trol is the only sample statistically different from all theothers (up to a significance level of p lt 0001) On thecontrary the other three samples (positive control and sam-ples polished with either VS and EN) are not significantlydifferent from each other even down to the less restrictivesignificance level of p lt 005 In Figure 3 the identified

difference has been pointed out by a line joining the com-pared groups

33 Mechanical Characterization

After the morphological roughness analysis by AFM thevalues of reduced elastic modulus Er and hardness H ofthe considered material have been measured on all thespecimens by means of microindentation Since opera-tion of this technique requires time consuming calibrationand thermal drift settling after each specimen replace-ment only one specimen of each sample was selectedfor these measurements The statistics has been providedby repeating indentations in 4 different positions on thesame specimen and by considering different loading-unloading cycles on each position In our measurementprogram 5 cycles have been performed with increasingmaximum load up to Fmax = 200 mN (in 40 mN steps)It can be observed from Figure 3 that this final maximumload corresponded to a final maximum indentation depthmax sim 5 mExtraction of the elastic properties in microindentation

critically relies on the so-called diamond area function(DAF)19 that is the actual shape of the diamond inden-ter tip which is stored in the instrument software and isused during the application of the Oliver-Pharr model20 tothe load-unload curves Since a sim5 m indentation depthis beyond our defined DAF the used Berkovitch pyramidcan not be described accurately any more and the actualindenter shape rather starts to be conical at this depthTherefore we decided to limit our quantitative analysis tothe first 2 cycles performed at the lowest maximum loadsup to Fmax = 80 mN This limit corresponded to a max-imum indentation depth of max sim 26 m (see secondcycle in Fig 4) which should still be high enough to fullypenetrate the microasperities on the surface even for theroughest specimens (positive control sample) which haveminimum to maximum range overcoming 1 m distance(see vertical scale of Fig 2(b)) If this would not be thecase no true full surface contact would be warranted dur-ing the microindentations leading to underestimation ofEr and especially of H

The results of the microindentations (means plusmn1 stan-dard deviation for both Er and H ) for the first two cyclesin Figure 4 (n = 8) are shown in Figure 5(a) In order tofacilitate the comparison of the values measured for thesurface treated samples we selected the negative control asa reference and used a scale for H such that the Er andH values for this sample appear at the same height levelwhich has been traced as a dotted line across the wholerange of different samples In Figure 5(a) it can be seenthat the mean values of Er and H for EN lie around the ref-erence line and the positive control values deviates pref-erentially towards higher values However in both casesdeviations are lower than the standard deviation (ie the



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Fig 4 Typical 5 cycles microindentation program of load-depth mea-surements performed The final maximum load has been increased lin-early from 40 to 200 mN in steps of 40 mN The first two cycles (Fmax =40 80 mN) can be used for evaluation of Er H in a limit where theindenter tip shape is perfectly modeled The last two cycles (Fmax = 160200 mN) can be used instead in a limit where the surface effects can beexcluded

error bar length) On the contrary the means of VS deviatetowards lower values beyond the extension of the respec-tive error bars In fact whereas for both positive controland EN the error bar lengths are increased with respectto the negative control which is expected after the scat-tering of the zero contact point due to increased surfaceroughness this is not the case for VS for which the errorbars are approximately as long as for the negative controlObviously the surface resulting from VS polishing is notonly smoother than that after EN polishing (Fig 3) butalso more uniform from the point of view of the elasticproperties As a consequence the decrease in Er and Hobserved for VS seems to be significant and not just dueto occasional statistical fluctuationsThe data in Figure 5(a) have been analyzed with

ANOVA For both Er and H no statistically significantdifference was observed for any pair comparison betweennegative control positive control and EN polished sam-ples However for H only the VS polished sample turnedout to be different from the positive control even if atrelatively low level of significance (p lt 005) If real thedecrease in elastic properties observed in Figure 5(a) forVS appears to be minus13 for Er and minus22 for H withrespect to the negative controlThe effect of mechanical surface treatments on the elas-

tic properties of a material can have several sources Formetals for example work hardening is often observedwhich brings in an increase in H contrary to the presentcase However the effect of polishing on a polymer com-posite is not as straightforward to interpret A decrease inH could possibly occur upon local heating resulting in a

(a)

(b)

Fig 5 Elastic properties (reduced modulus Er and hardness H )obtained by means of microindentation on the differently treated sam-ples (n= 8) (a) Using the 2 cycles at lowest maximum load (Fmax = 4080 mN) and (b) using the 2 cycles at highest maximum load (Fmax = 160200 mN)

so-called thermal insult In fact our specimens were milledand polished soon after their photopolymerization iebefore complete resin composite maturation which couldhave affected their behavior21 However other authors haveshown that immediate polishing does not always affect theelastic properties of the restorative composite with respectto delayed polishing22 but of course this may criticallydepend on the type of material For example polishinghas been found not to affect H of metallic orthodon-tic archwires23 whereas it decreased the H of dentalamalgam24

Another possible reason for the apparent decrease in theelastic properties can be an artifact in the measuring tech-nique associated with the evaluation of the actual contactarea Ac between indenter tip and specimen This couldalso be the reason for the higher deviations observed inFigure 5(a) for H with respect to Er for both EN andVS samples since it is known that Er sim 1

radicAc whereas

H sim 1Ac and the latter is therefore more sensitive touncertainty in the Ac parameter25

In microindentation several models have been workedout to try to account for the effect of surface roughness of



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LEthe specimens2627 The simplest model describes the mea-sured hardness H as a function of the ideally measuredhardness on a flat surface H0 under the hypothesis thatthis be the bulk material hardness with a phenomenolog-ical equation of the type HH0 = 1minus e

n where e

is an error term affecting indentation depth due to theroughness and n is a parameter depending on the inden-ter geometry For a conicalpyramidal indenter as in thepresent case n= 228 and as an estimate of e the surfaceRMS roughness can be used It can be seen that for agiven this model assigns a decrease in measured H forthe roughest surfaces Therefore a decrease in H for VSwith respect to the negative control would be qualitativelyjustified but it would not be possible to understand whythe positive control and the EN sample show higher Hthan VS whereas their RMS roughness is higher On theother hand the so-called indentation size effect29 is knownto overestimate H at very small which is often asso-ciated to a bad determination of the zero contact point27

In both cases an effect above sim 2 m should hardly beobserved in the absence of a real material non-uniformityIn order to check if in our case the observed H differ-

ence for VS was due to the measurement or to a real mate-rial property we have then examined also the last 2 cyclesof our microindentations performed with the highest max-imum loads Fmax = 160 and 200 mN From these cycleswith the same statistics as the previous analysis (n = 8)independent values of Er and H have been extracted andare reported in Figure 5(b) Again a dotted line repre-sents the reference for both Er and H with respect to thenegative control For the sake of comparison the dashedline in Figure 5(b) represents also the level of the dottedline in Figure 5(a) The vertical scales are the same forboth panels (a) and (b) of Figure 5 It can be seen imme-diately that by increasing the indentation depth lowervalues are measured for both Er and H for all samplesHowever we have previously discussed that due to theexcessive max sim 5 m the absolute values of the Er andH obtained from these cycles can be inexact as the DAFis not as expected In any case it is possible to examinethese data to compare again the relative values of Er andH measured for the different samples One can see fromFigure 5(b) that whereas the VS values are again lesserthan the controls in this case also the EN values devi-ate significantly in the same direction Furthermore a newANOVA analysis for these datasets showed no significantdifference in any pair of samples for any quantity (eitherEr or H ) Therefore we can conclude that the significantlylower H previously observed for VS with respect to thepositive control was not due to a real bulk property of thecompositeOverall when considering all the measurements of

Figure 5(a) as a single group only representing the non-modified VD material Er and H were measured to be(127plusmn 20) GPa and (435plusmn 105) MPa with a relativeuncertainy of 16 and 24 respectively (n= 32)

For the possible explanation of the difference observedin H (and Er for VS in Figure 5(a) we speculate thatthis could be due to smear composite material displacedby the VS polishing system from the mountains into thevalleys of the previously rougher material thus planarizingit but making the surface hardness to appear lower at thecomparatively low max considered for the first two cyclesat lowest maximum loads

34 Surface Composition

In order to check for possible surface contamination alsopotentially affecting our surface measurements of the elas-tic properties SEM imaging was integrated with EDSmeasurements To this goal from low magnification SEMimages of the samples similar to the areas shown inFigure 1 the corresponding EDS microanalysis has beenperformed and typical spectra are presented in Figure 6In all the spectra all the elemental species expected toappear in VD are observed with a dominating content ofthe Si and O from the glass (mainly silica SiO2 filler par-ticles and Al and Ba as the glass filler contained metals16

Additionally but to a lower extent (as it is sim36 in vol-ume) C from the polymeric resin matrix is presentInteresting differences can be observed among the sam-

ples The amount of C is clearly highest in the nega-tive control (as cured composite Fig 6(a)) In the othersamples the atomic population of C decreases mostly(minus20) for the positive control (as milled sample) dueto the strong abrasive action of milling that predominantlyremoved the softer matrix in between the surface filler par-ticles In the polished samples the amount of C is alsoslightly lower than the negative control (minus3 for EN andminus9 for VS) yet is almost completely restored to theoriginal ratio to the other elements Probably the filler par-ticles have been preferentially removed from the surfaceupon polishing after milling and the remaining C is notfurther removed but is rather displaced around Accord-ingly most of the other elements (Al Si and Ba) increasedwith respect to the previous quantities of between +9and +17 whereas O remained approximately the same(+3 probably due to removal of surface moisture)The only other element whose contents is changing sig-

nificantly among the different spectra (all elements chang-ing between minus7 and +5 only) is Al which is doubled inEN (+103 Fig 6(d)) Obviously the ratio to the otherelements in the glass filler particles not being the samewith respect to the negative control this Al is coming fromoutside the VD In fact one can assign it to alumina abra-sive particles from the EN polishing slurry which haveprobably stuck to the treated surface This could be thereason for the higher H observed for EN in Figure 5(a)which is removed when indenting deeper into the real VDmaterial under the top surface in Figure 5(b) For the VSsample on the contrary the EDS spectra show no con-tamination from the polishing abrasive (Si based) In this



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LEFig 6 Representative EDS spectra of the differently treated samples (a) negative and (b) positive controls and samples polished with (c) VS and(d) EN Extracted from 2000times image areas as in Figure 1

case we still think that the valley filling effect with partlytorn out material during polishing can be the reason forthe observed decrease in H at the low indentation depthregime as observed in Figure 5(a)

4 SUMMARY AND CONCLUSIONS

We have investigated the possible consequences of surfacepolishing of a dental material on its elastic properties Thesamples treated in different ways (as cured and as milledmaterial called negative and positive control respectivelyas well as polished with two different methods EN andVS) have been first investigated by SEM and AFM whichallowed to characterize the surface morphology and rough-ness Then we have performed microindentation mea-suring the compressive modulus and the hardness of thesamples At sim2 m indentation depth the elastic propertiesshowed a significant dependence on the surface treatmentin the case of VS polishing which resulted in a decreaseof both modulus and hardness of sim10 and sim20 respec-tively We assigned this effect to the possible accumulation

of smear in the valleys of the originally rougher mate-rial Indeed by measuring at higher indentation depth ofsim5 m this difference disappears The effect of surfacecontamination by the abrading particles used in the pol-ishing systems has also been addressed by EDS micro-analysis This analysis could justify the slight increase inhardness of the EN sample as probably due to abrasivealumina particles loosely bonded to the material surfaceWe conclude that no real modification of the material prop-erties is observed on VD for the polishing treatments apartfrom temporary contamination or accumulation of surfacesmear This finding can be of clinical relevance for the den-tal practitioners who can apply the considered treatmentson the investigated material without risking to degrade itselastic properties Future investigations in our laboratorywill be directed towards a nanoscopic mechanical analysisby means of AFM nanoindentation

Acknowledgments Mr Mario Malerba of the Tech-nical staff of the Electron Microscopy laboratory at theNanochemistry Unit of IIT is gratefully acknowledged for



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LEhis help during SEM imaging and EDS analysis of thesamples

References and Notes

1 J Millman and A Grabel (eds) Microelectronics 2nd EditionMcGraw-Hill New York (1995)

2 P Rai-Choudhury (ed) Handbook of Microlithography Micro-machining and Microfabrication Microlithography SPIE PressBellingham Washington (1997)

3 M C Gupta and J Ballato Handbook of Photonics CRC PressTaylor and Francis Group Boca Raton (2007)

4 M Salerno J Krenn B Lamprecht G Schider H DitlbacherN Feacutelidj A Leitner and F Aussenegg Opto-electronics Rev10 217 (2002)

5 C T Leondes (ed) MEMS-NEMS Handbook Springer New York(2006)

6 Y Xia and G M Whitesides Annu Rev Mater Sci 28 153 (1998)7 W Teughels N Van Assche I Sliepen and M Quirynen Clin Oral

Imp Res 17 68 (2006)8 V Uskokovic and L E Bertassoni Materials 3 1674 (2010)9 S Sharma S E Cross C Hsueh R P Wali A Z Stieg and J K

Gimzewski Int J Mol Sci 11 2523 (2010)10 M Salerno L Giacomelli G Derchi N Patra and A Diaspro

Biomedical Engin Online 9 59 (2010)11 R Gedik F Huumlrmuumlzluuml A Coskun Ouml Ouml Bektas and A K

Oumlzdemir J Am Dent Assoc 136 1106 (2005)12 T Watanabe M Miyazaki T Takamizawa H Kurokawa A Rikuta

and S Ando J Oral Sci 47 21 (2005)13 J Janus G Fauxpoint Y Arntz H Pelletier and O Etienne Dent

Mater 26 416 (2010)

14 L Giacomelli G Derchi A Frustaci O Bruno U CovaniA Barone D De Santis and F Chiappelli The Open DentistryJournal 4 191 (2010)

15 Heraeus Kulzer company web site pages on the product httpwwwheraeus-venuscomenusaproducts_10venusdiamond_10venusdiamond_3html

16 J Ferracane Resin composite state of the art Proceedings of theAcademy of Dental Materials Annual Meeting Trieste Italy October(2010)

17 Venus diamond instructions for use leaflet included in the productpackage Heraeus Kulzer Hanaus Germany

18 K Z Kantorski R Scotti L F Valandro M A Bottino C YKoga-Ito and A O Jorge Oral Health Prev Dent 7 29 (2009)

19 A C Fischer-Cripps Nanoindentation Springer New York(2004)

20 W C Oliver and G M Pharr J Mater Res 7 1564 (1992)21 G C Lopes M Franke and H P Maia J Prosthetic Dentistry

88 32 (2002)22 D Venturini M S Cenci F F Demarco G B Camacho and J M

Powers Operative Dentistry 31 11 (2006)23 N P Hunt S J Cunningham and C G Golden Angle Orthod

69 433 (1999)24 S A Ribeiro T N Do Nascimento A L B Centola L C Teixeira

and S M Campos Braz Dent J 2 135 (1991)25 J Hay Experimental Techniques 33 66 (2009)26 M S Bobji and S K Biswas J Mater Res 13 3227 (1998)27 G B de Souza C E Foerster S L Rutz da Silva and C M

Lepienski Materials Res 9 159 (2006)28 H J Weiss Phys Status Solidi A 129 167 (1992)29 G M Pharr E G Herbert and Y Gao Annual Review of Materials

Res 40 271 (2010)



IP 9014726244Wed 11 Apr 2012 085835


LEfocused our attention mainly on the elastic mechani-cal properties of the polished composite Additionallythe experimental analysis of the sample surfaces hasbeen complemented with SEM microanalysis via energy-dispersive X-ray spectroscopy (EDS)

2 MATERIALS AND METHODS

21 Sample Preparation

The resin composite of choice is Venus Diamond (HeraeusKulzer Duumlbendorf Germany) shortly VD in the follow-ing VD is a dental restorative material available on theEuropean market since approximately two years which isinnovative in both organic matrix formulation and fillerparticles15 According to a recent filler size-type rankingof resin dental composites VD can be termed as a lsquomid-ifillrsquo in that it contains both nanofillers and microfillersin a diameter range of 5 nmndash20 m1617 The filler parti-cles are made of Barium Aluminium Fluoride glass andthe nominal filler loading is 64 by volume16 The interestof our group has recently focused on VD as an exemplarymaterial for universal restorations ie for both anteriorand posterior teeth of which we aim to deepen our cur-rent understanding of the basic properties in order to beable to associate them with correct clinical performanceexpectations10

211 Specimen Fabrication

Square specimens of VD (sim10times10 mm2 sim15 mm thick-ness) were prepared by compressing the composite resinpaste into a polyethylene hollow matrix with an amalgamcondenser and removing the excess material The speci-mens were then irradiated through a transparent acrylatestripe for 40 s with a blue (440ndash480 nm wavelength) LEDcuring lamp Translux Power Blue (Heraeus Kulzer) withan irradiance of sim1 Wcm2 16 specimens of VD wereprepared that were subdivided into 4 groups dedicated todifferent surface treatments namely the two polishing sys-tems and a negative and a positive control

212 Specimen Polishing

Two different polishing systems were tested namelyVenus Supra (Heraeus Kulzer) and Enhance (DentsplyCaulk DE USA) shortly VS and EN in the followingThese polishing systems were selected after their best over-all performance (ie lowest resulting surface roughness) ina group of three polishing systems investigated in a pre-vious study14 VS is a one-step polishing system basedon silicone-impregnated polishing points EN is a multi-step polishing system combining pointed shape polishingpoints with a polishing cup with abrasive paste with overallalumina abrading particles in the range of 03ndash1 m diame-ter As a negative control a non-polished non-milled sample

has been used Additionally as a positive control a milledbut non-polished sample has been considered These sam-ples are called lsquonegativersquo and lsquopositiversquo control since theirresulting roughness is expected to be lesser or larger thanthat of the polished samples respectivelyThe polishing procedure was performed always by the

same expert operator according to the manufacturerrsquosinstructions with a process time of 20 s to reproduce theclinical practice After polishing the samples were blownwith air and stored in air at room temperature for one weekbefore the analysis For each sample four specimens wereprepared and analyzed

22 SEM Analysis

SEM measurements were carried out at 15 kV electronbeam acceleration voltage with a JSM-6490LA (JEOLLtd Tokyo Japan) collecting secondary electron emissionimages of the composite sample surface at both 2000 and10000 magnification (X) The samples did not require anymetal or carbon coating thanks to the low vacuum operat-ing conditions (25times10minus1 mbar) In some cases after SEMimaging EDS was also performed for identification of theabundance of elemental atomic species on the material sur-face on smaller selected areas of interest

23 AFM Analysis

The measurements have been taken with a commercialMFP-3D AFM (Asylum Research Santa Barbara USA)The relative height maps have been acquired in air intapping mode with silicon probes NSG10 (NT-MDTZelenograd Russia) which had nominal spring constantand fundamental resonance frequency values of sim10 Nmand sim250 kHz respectively The roughness of each spec-imen has been evaluated as the root mean square value(RMS) of the distribution of heights in the AFM topo-graphical images Optimum scan area was estimated to be30times 30 m2 (ie 30 m scan size) chosen such as tobe representative of a region typically affected by attach-ment of several oral bacteria adhering to the dental com-posite surface when performing in vivo18 For each sample4 specimens were measured in 3 different regions each(n= 12)

24 Microindentation

Microindentation experiments were performed by usinga NanoTest setup (Micro Materials Ltd Wrexham UK)equipped with a Berkovich three-sided pyramid diamondindenter with nominal radius of curvature at the tip ofsim50 nm The indentations were performed in a cabi-net with constant temperature of 23 C in mechanicaland electrical low-noise conditions under load-controlThe setup was first calibrated by five iterative indentationcycles into fused silica (with maximum load increasing



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Documents

Combined Effect of Polishing on Surface Morphology and Elastic Properties of a Commercial Dental Restorative Resin Composite