Surface roughness and morphology of three nanocomposites after two different polishing treatments by a multitechnique approach

d e n t a l m a t e r i a l s 2 6 ( 2 0 1 0 ) 416–425

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Surface roughness and morphology of three nanocomposites after two different polishing treatments by a multitechnique approach J. Janus a , G. Fauxpoint a , Y. Arntz c , H. Pelletier b , O. Etienne a,∗ a b c

School of Dental Medicine, University of Strasbourg (UDS), France National Institute for Applied Sciences (INSA), Strasbourg, France School of Dental Medicine, Inserm U-595, University of Strasbourg (UDS), France

a r t i c l e

i n f o

a b s t r a c t

Article history:

Objectives. The purpose of this study was to assess the surface roughness and morphology

Received 19 April 2009

of three nanocomposites polished with two different polishing systems.

Received in revised form

Methods. Specimens made of hybrid composite (Tetric Ceram [TC] as control) and nanocom-

19 July 2009

posites: nanofilled (Filtek Supreme [FS]), nanofilled hybrid (Grandio [Gr]), complex nanofilled

Accepted 10 September 2009

hybrid (Synergy D6 [Syn]) were polished with CompoSystem [CS] or Sof-Lex [SL] polishing discs. The average surface roughness (Ra) before and after polishing was measured using optical profilometry. Both AFM and SEM techniques were additionally used to analyze the

Keywords:

surface morphology after polishing with the aim of relating the surface morphology and

Nanocomposite

the surface roughness. Statistical analysis was done by ANOVA using a general linear model

Nanoparticles

(˛ = 0.05) with an adjustment for multiple comparisons.

Polishing

Results. Within the same polishing system, FS exhibited the smoothest surface, followed by

Surface

Syn, TC and Gr (p < 0.0001). Sof-Lex polishing discs produced the smoothest surface com-

Roughness

pared to CompoSystem (p < 0.0001). AFM and SEM observations confirmed that the surface

AFM

roughness was related to the surface morphology and to the average filler size.

Profilometry

Significance. Positive correlation between the average filler size and the surface roughness

SEM

suggest that using nanoparticles in the formulation does not necessary improve the surface texture. The nanofilled composite FS, which contains only nanofillers, showed the best results when associated to Sof-Lex polishing discs. © 2010 Academy of Dental Materials. Published by Elsevier Ltd. All rights reserved.

1.

Introduction

Development and advances in the field of filler technology have affected dentistry since the introduction of dental resin composites about forty years ago. Since that time composites with macro-, micro- and nanofiller particles have been suc-

cessively proposed. Today, only few macrofilled composites are still available in the dental market because of their inadequate surface texture. Compared to macrofilled composites, microfilled composites show better surface properties as well as superior aesthetic qualities, however their poor mechanical properties restrict their usage to non-stress-bearing areas. Microhybrid composites are most widely used as they provide

∗ Corresponding author at: Prosthodontics Department, School of Dental Medicine, 1, Place de l’Hopital, 67000 Strasbourg, France. Tel.: +33 388 320 329. E-mail address: [email protected] (O. Etienne). 0109-5641/$ – see front matter © 2010 Academy of Dental Materials. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.dental.2009.09.014

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optimal mechanical and physical properties combined with good polishing properties. However, one of the most important advances of the last few years in the field of filler technology is the application of nanotechnology to dental composites. Nanofillers have been developed with the aim of combining the advantages of hybrid and microfilled composites in the same restorative material. Nanofillers are described as “the discrete particles which have all of three dimensions in the range of about 1–100 nm” [1,2]. Nanocomposites built with these nanofillers have a low shrinkage attributed to the high filler volume loading [3,4]. They show favorable mechanical properties, which are at least equal to or may surpass those of hybrid materials [4–11]. They exhibit a higher surface quality, a better polish and gloss, an increased retention as well as an increased wear resistance [4,5,12–18]. Among these properties, surface roughness is greatly taken into consideration. Roughness has a major impact on the aesthetic appearance and discoloration of restorations [18–27], plaque accumulation, secondary caries and gingival irritation [28,29], and wear of opposing and adjacent teeth [30]. In addition, a smooth surface ensures patient comfort and facilitates oral hygiene [30]. The smoothest composite surface, as known, is obtained under polyester matrix film [12,15,32–34]. Nevertheless, removal of the resin rich surface layer is necessary because of compromising mechanical properties [35], biocompatibility [36,37] and increasing staining [24]. Furthermore, the surface treatment is systematic, following the placement of composite in order to remove excess material, to adjust the anatomic form and the occlusion, and, finally, to obtain a smooth surface. For these purposes, a wide variety of finishing and polishing instruments are available. Among them, aluminum oxide graded abrasive flexible disks were reported to produce the best surface smoothness [30,38–40]. The purpose of the present study was to investigate the surface roughness and the surface morphology of three dental composites containing nanoparticles and one hybrid composite after polishing with two different aluminum oxidebased polishing systems. The formulated hypotheses were: (i) Is there is a significant improvement in surface roughness between nano and hybrid composites? (ii) Is the surface roughness of the tested composites related to their surface morphology? (iii) Is the surface roughness after polishing significantly different depending on the polishing system?

2.

Materials and methods

2.1.

Materials and preparation of the specimens

were divided into two groups. Each group was polished using one polishing system. Each abrasive disk was used only once, under dry conditions, for 20 s, using a slow-speed handpiece at a speed according to manufacturer’s instructions.

2.2.

Surface roughness analysis

Surface roughness of each composite was assessed quantitatively by optical profilometry with a Chromatic confocal point sensor CHR 150-N (Stil, Aix en Provence, France) with an optical pen of 300 ␮m. Each surface was scanned by eleven parallel tracings (length = 1 mm) per area of 1 mm × 1 mm. One area per specimen was analyzed for the samples without any polishing treatment, whereas two areas were analyzed for the polished surfaces. Measurement areas were chosen randomly, excluding a surface of 1 mm from the edge, which was not representative of the polishing. The average surface roughness (Ra) of each specimen was calculated with a cut-off value of 0.08 mm. One representative zone of 0.3 mm × 0.3 mm of each composite and of each surface treatment (matrix, Sof-Lex, CompoSystem) was scanned by 151 parallel tracings to give a 3D reconstructed image.

2.3.

Atomic force microscopy observations

Due to the nature of the composites, AFM microscopy was required to resolve nanocharges present in the material. The surface and filler morphology of the composites tested was investigated using contact mode with a commercial atomic force microscope (AFM) Nanoscope III with controller IV from Digital Instruments (Veeco Metrology Inc., Santa Barbara, CA, USA). Cantilevers with a constant spring of 0.1 N/m and with silicon nitride tips of 20 nm radius, were used (model MLCT-AUHW Park Scientific, Sunnyvale, CA, USA). Several scans over a given surface area were performed to provide reproducible images. Deflection and height mode images were obtained simultaneously at a fixed scan rate (between 1 and 2 Hz) with a resolution of 512 × 512 pixels. Images were acquired with 20 ␮m × 20 ␮m, 10 ␮m × 10 ␮m and 5 ␮m × 5 ␮m sizes. The images were analyzed with specific softwares (Nanoscope v613r1, Veeco Metrology Inc., Santa Barbara, CA, USA and WSxM 4.0 Develop11.1, Nanotec Electronica S.L., Tres Cantos, Spain).

2.4.

Three nanocomposites, one hybrid composite and two different polishing systems were tested (Table 1). Twelve specimens of each material were made using cylindrical metallic molds (5 mm diameter × 3 mm depth). A transparent matrix strip was applied at the top of the surface of each composite with a constant pressure to extrude excess material, to flatten the surface and to reduce voids at the surface. Specimens were then polymerized according to the manufacturer’s recommendations with a Heliolux DLX (Vivadent, Schaan, Liechtenstein). After curing, all specimens were transferred to other cylindrical metallic molds (5 mm diameter × 2.5 mm depth) and

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Scanning electron microscopy observations

Representative specimens of each group of polished composites were analyzed qualitatively by scanning electron microscope (SEM) (XL 30 ESEM FEI Company, Hillsboro, Oregon, USA), following a sputter gold coating (460 Å thickness) (Edwards S 150 Sputter Coater, Edwards High Vacuum International, Wilmington, Massachusetts, USA).

2.5.

Statistical analysis

All profilometry results were analyzed by means of two-way analysis of variance (ANOVA, SAS 9.1, SAS Institute, Cary, NC, USA) using a general linear model at a significance level of ˛ = 0.05 with an adjustment for multiple comparisons.

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Table 1 – Details of the tested materials: bis-GMA: bisphenol-A glycidilmethacrylate; bis-EMA: ethoxylatedbis-phenol-A dimethacrylate; UDMA: urethane dimethacrylate; TEGDMA: triethyleneglycol dimethacrylate. Material (group)

Type

Shade

Matrix

Filler type and size

Filler average size

Filler loading vol%

Manufacturer

Batch#

wt%

List of tested materials

Filtek Supreme XT (FS)

A3B

Nanofilled

Bis-GMA Bis-EMA UDMA TEGDMA

Grandio (Gr)

Nanofilled hybrid

A2

Bis-GMA Dimethacrylate

Bis-GMA Synergy D6 (Syn)

Complex nanofilled hybrid

A3,5/B3 Bis-EMA

UDMA TEGDMA

Tetric Ceram (TC)

Hybrid

B3

Bis-GMA

UDMA TEGDMA

Polishing system List of polishing systems investigated Sof-Lex Discs (SL) 9.5 mm diameter

CompoSystem (CS) discs 9 mm diameter

Type

Composition

0.6–1.4 ␮m

Ba–Al–borosilicate glass filler Nanofiller (SiO2 )

1 ␮m

PPF (prepolymerized fillers) Barium glass filler <2.5 ␮m

20 ␮m

Microfiller (SiO2 aggregated) Nanofiller (SiO2 )

150 nm

Filler average size

Speed (rpm)

78.5

3M ESPE (St. Paul, MN, USA)

5CT

71.4

87

Voco (Cuxhaven, Germany)

700173

65

80

Coltene Whaledent AG (Altstatten, Switzerland)

89438

60

75

Ivoclar Vivadent AG (Schaan, Liechtenstein)

G08059

20 nm

20–50 nm

0.6 ␮m

20–80 nm

Barium glass 0.7 ␮m filler Ba–Al–fluoroborosilicate glass filler Microfiller (SiO2 ) Mixed oxide (ZrO2 /SiO2 ) Ytterbium trifluorure

59.5

40 nm 0.2 ␮m

Manufacturer

Coarse Medium Fine Superfine

Al2 O3 Al2 O3 Al2 O3 Al2 O3

92–98 ␮m 25–29 ␮m 16–21 ␮m 2–5 ␮m

10,000 10,000 30,000 30,000

3M ESPE Dental Products (St. Paul, MN, USA)

Medium Fine Ultrafine

Al2 O3 Al2 O3 Al2 O3

50 ␮m 30 ␮m 5 ␮m

10,000 10,000 10,000

Komet, Gebr. Brasseler GmbH&Co (Lemgo, Germany)

Batch# P061121

32788 33452 32393

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UDMA TEGDMA

Zirconia–silica cluster filler Nanofillers (SiO2 )

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Fig. 1 – 3D roughness reconstructed images (scan size 0.3 mm × 0.3 mm, ˛ and ˇ angles = 30◦ ) of the tested composites obtained by optical profilometry. Unpolished surfaces (left) showed few air voids (black arrows) and reproduction of matrix imperfections (white arrow). Polished specimens presented a rougher surface and some grooves left by the abrasives. CompoSystem discs (middle) and Sof-Lex discs (right) produced similar roughness within each composite group. The smoothest polished surface was observed for Filtek Supreme whereas Grandio exhibited the roughest.

Table 2 – Mean surface roughness Ra (␮m) and standard deviations (SD) of investigated composites. Polishing system

Composite Filtek Supreme

Grandio

0.025 (0.003) 0.094 (0.007) 0.123 (0.01) 0.11 (0.017)

0.017* (0.004) 0.162 (0.02) 0.212 (0.017) 0.191 (0.03)

Matrix surface SL discs CS discs All systems (SL, CS)

Synergy D6 0.018* (0.003) 0.109 (0.01) 0.133 (0.01) 0.12 (0.015)

Tetric Ceram 0.017* (0.002) 0.12 (0.009) 0.14 (0.011) 0.133 (0.016)

All composites 0.02 (0.004) 0.119 (0.022) 0.15 (0.038) 0.136 (0.037)

Average roughness (standard deviation) of the tested materials as obtained by optical profilometry. ∗ All results were statistically different except (*). See also Fig. 3 and comments in text.

3.

Results

3.1.

Surface roughness analysis

Qualitative and quantitative analyses of the surface roughness were assessed with optical profilometry. Fig. 1 illustrates representative 3D-constructed surface images, showing an increasing roughness after polishing. Mean values of Ra and standard deviations produced by Matrix strip, Sof-Lex discs and CompoSystem discs of the four resin composites tested are reported in Table 2 and Fig. 2.

3.2.

Global comparisons

One-way ANOVA and multivariate analysis showed a significant difference (p < 0.0001) between all composites and pol-

Fig. 2 – Average roughness (boxes: standard deviation, ×: lower and higher quartile) of the materials tested as obtained by optical profilometry. All results were significantly different except ( ).

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Fig. 3 – AFM images of polished surfaces of resin composites: (A) Tetric Ceram; (B) Filtek Supreme; (C) Grandio; (D) Synergy D6. Deflection-images (scan size 10 ␮m × 10 ␮m) give the overall view of the surface showing the filler size and distribution; 3D images (scan size 5 ␮m × 5 ␮m) show the protruding or plucked out fillers (arrows).

ishing systems. A significant statistical interaction (p < 0.0001) between composite and polishing system was noticed and required the use of the nested model. The percentage of variance (R square value) explained by the model was 70% for composites only, 19% for polishing system only and 90% for the nested model, showing a high reliability of the multivariate model.

3.3.

Multiple comparisons

As there was a significant interaction between composites and polishing systems, multiple comparisons were made according to the nested model. The smoothest surfaces for all the materials were obtained against the matrix strip. Filtek Supreme group exhibited the

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lowest roughness values (p < 0.0001), while no statistically significant differences were observed among the three other materials. All the resin composites showed a significant increase in roughness of the surface after polishing procedures (p < 0.0001). Both techniques (Sof-Lex and CompoSystem) produced the smoothest surfaces for Filtek Supreme XT, followed by Synergy D6, Tetric Ceram and Grandio having the highest Ra values. Statistically significant differences were observed among them (p < 0.0001).

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Among the polishing systems, the lowest Ra values for all resin composites were attributed for the Sof-Lex system (p < 0.0001).

3.4.

Atomic force microscope observations

According to the AFM images, there were no observed differences in the surfaces polished by the two devices. Therefore, the AFM images were chosen, regardless of polishing technique, in order to represent the surface morphology of

Fig. 4 – SEM images (high magnification views) of the composites surfaces polished with CompoSystem discs (left) and with Sof-Lex discs (right): (A) Tetric Ceram; (B) Filtek Supreme; (C) Grandio; (D) Synergy D6 (black arrows designate prepolymerized fillers; white arrows indicate the voids left by plucked out fillers).

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Table 3 – Ra values as obtained by AFM with scan size of 20 ␮m × 20 ␮m.

4.

TC

Resin composite materials tested in this study were selected in order to represent all three of the nanoparticle dental composite subclasses and Tetric Ceram as microhybrid reference material. Filtek Supreme XT filler structure is close to the microfilled composites structure and therefore can be classified in the nanofilled composites subclass. The presence of macrofillers as well as nanofillers in the composition of Grandio allows its classification in the nanofilled hybrid composite subclass. Finally, the presence of prepolymerized fillers (PPF) in addition to macro- and nanofillers put Synergy D6 among the complex (or blended) nanofilled hybrid composites subclass. To fully characterize surface texture of the composites investigated, a multitechnic approach was employed as Silikas et al. or Teixeira et al. suggested [16,27]. An optical profilometry was performed to measure the surface roughness of tested composites. Both SEM and AFM techniques were used to qualitatively assess surface texture and surface morphology of the specimens. According to the optical profilometry, the smoothest surfaces were observed when the specimens were cured against a polyester film. Unpolished surfaces of all tested composites were significantly smoother than polished specimens. Filtek Supreme exhibited the highest roughness value (p < 0.0001) while the roughness of other composites was not significantly different. Since the resin matrix of the investigated specimens was not fundamentally different, other factors like filler features might be involved. Glass fillers of irregular forms found in Grandio, Synergy D6 and Tetric Ceram may have enabled an easier compaction during placement and a better extrusion of the resin compared to the round clusters found in Filtek Supreme. However, this resin rich layer has poor physical, mechanical and biological properties. Therefore it should be eliminated during finishing-polishing procedures [24,35–37]. Moreover, its roughness would be exclusively assigned to the contribution of the resin matrix [27] and of the matrix intrados imperfections [27,34,41]. After polishing procedures, the profilometry results (Table 2) indicated that not all of the three nanocomposites investigated had a lower surface roughness compared to the hybrid composite. Thus, the first hypothesis suggesting that there is a significant improvement in surface roughness between nano and hybrid composites can only be partially accepted. This finding is supported by other studies [13,15,21], which concluded that composites containing nanoparticles did not constitute a homogeneous group regarding surface roughness after polishing. The same heterogeneous observations have been noted for their mechanical properties [5,6,8,17]. According to the surface roughness assessment, Filtek Supreme showed the smoothest surface after both polishing techniques used in the present study (p < 0.0001). This observation is in agreement with the results of previous studies where surface roughness of Filtek Supreme after polishing with Al2 O3 abrasive grated discs was less than that of hybrid composites [12,15,27] and similar to that of microfilled composites [12,41].

FS

GR

Syn

CS

SL

CS

SL

CS

SL

CS

SL

0.101

0.082

0.043

0.025

0.094

0.185

0.069

0.077

the specimens caused by the exposed fillers. Representative images of the resin composites investigated are shown in Fig. 3. The Filtek Supreme surface displayed the smoothest surface with a homogeneous distribution of rounded particles organized in clusters (0.56–1.11 ␮m). The deflection-image of Synergy D6 also showed the homogeneous distribution of the glass filler particles (0.54–1.07 ␮m). However, the 3D images revealed that the surface relief was more pronounced. Fig. 3A and C showed the most voluminous glass fillers exposed at the surface of Grandio and Tetric Ceram. The largest glass particles were observed for Grandio (1.50–3.20 ␮m) followed by Tetric Ceram (1.02–2.10 ␮m). Both materials also presented filler particles of intermediate size (0.49–0.92 ␮m). Depression areas between the glass particles were observed, indicating the removal of resin with nano or microfillers and/or dislodgment of the glass particles during polishing. High magnification 3D images (Fig. 3, right) revealed smaller discrete particles (<120 nm) filling the interstice between the glass filler for all groups except Tetric Ceram. However, AFM did not differentiate whether the particles were discretely dispersed or aggregated. The average roughness (Ra) of the polished specimens (scan size 20 ␮m × 20 ␮m), obtained by AFM, is reported in Table 3. Ra values were calculated once for each polishing condition and specimen. As the area was reduced, no statistical comparisons were conducted. However, these Ra values were useful to correlate surface roughness and filler sizes and distribution (see Section 4).

3.5.

Scanning electron microscope observations

SEM images of polished specimens are shown in Fig. 4. The low magnification views showed smooth surfaces for all the composites and polishing techniques, except for Grandio, which presented visible voids. All of the composites (except Grandio) presented scratch lines for the two polishing systems. The highest magnification images revealed the smoothest surface for Filtek Supreme following by Synergy D6, Grandio and Tetric Ceram, which had surface irregularities of varying degrees. Grandio showed multiple filler dislodgements of the greatest sizes. A large scratch line probably left by the tip of the mandrel was observed at the Tetric Ceram surface polished with the Sof-Lex discs, whereas the remains of its surface was more homogeneous with voids of a smaller size. Only very few voids were observed at the Filtek Supreme surface the size of which was in the range of the cluster fillers.

Discussion

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This result could be related to the specific composition of Filtek Supreme, which contains only nanofillers, which is in the same size range as the microfillers. The nanofillers are discreetly dispersed or organized in clusters. These purely inorganic clusters are formed by individual primary nanoparticles bonded between them by weak intermolecular forces [42]. Hence, these nanoparticles may break away from the clusters during wear or polishing [17,25,43]. Ra values (Table 2) of Synergy D6 and Grandio were significantly lower and higher, respectively, than those of hybrid composite Tetric Ceram. This result is in accordance with previous published investigations in which the nanoparticle resin composites containing PPF exhibited the least surface roughness compared to the hybrid counterparts while the nanofilled hybrid composites were similar or rougher than the latter [13,14,22,26]. Most of the published studies investigating nanocomposites were based on mechanical profilometry. In those studies, Ra values were reported as 0.125–0.260 ␮m for Filtek Supreme polished with Al2 O3 grated abrasive discs [12,15,25,27,32,41], as 0.15–0.79 ␮m for Grandio [24,30] and as 0.169 ␮m for Tetric Ceram [27]. The corresponding values of the present study were comparable. Only one study using an optical laser device showed a doubling of roughness [14]. In the present study, all polished composites (except for Gr/SL group) exhibited a roughness below 0.2 ␮m, which is reported as the threshold of initial bacterial plaque accumulation by a review of the literature based on studies using mechanical profilometry devices [28]. In the current study an optical white light profilometry was used for assessing surface roughness of the composites as this had several advantages when compared to a mechanical method. The resolution of an optical white light sensor is much finer: around 10 nm for vertical resolution and 2 ␮m for lateral resolution, while that of a mechanical stylus is of 20–50 ␮m [22]. Thus, the roughness values obtained are more accurate, especially when viewing a very smooth surface like that of new nanocomposites [33]. Moreover, a non-contact acquisition excludes surfaces damages that could be caused by the mechanical sensor and that could consequently create a bias in the results obtained [44]. Since several authors [33,45] have found that there was a significant difference between the values obtained using these two methods, direct comparison of the Ra values from the present study with the results of other studies as well as with the reported threshold is delicate. Polishing is complicated by the heterogeneous nature of dental composites with both hard filler particles and soft resin matrix [15,32]. Resin removal rather than glass filler abrasion during the polishing procedure contributes to the exposure of filler particles and increases the surface roughness [21]. It is generally accepted that the polishability of resin composites greatly depends on size, shape, hardness and quantity of filler particles [21,26,31–33]. In the present study, the roughness data obtained from profilometry (Table 3) was positively correlated with the average sizes of the glass filler particles. However, no correlation was found between roughness and filler content contrary to Jung et al. [13]. The differences between filler size distribution of the materials were clearly observable from AFM images. As shown by the 3D-height images (Fig. 3), the most voluminous glass fillers of

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Grandio protruding from the surface could explain its highest roughness values. Conversely, the glass fillers of smaller size observed in Synergy D6 were uniformly distributed at the whole surface and therefore were positively affecting surface smoothness. The lowest Ra values obtained from both profilometry and AFM techniques for Filtek Supreme could be related to the homogeneous structure of its surface observed in Fig. 3. The high-resolution capacity of AFM permits accurate views of the surface topography, with 3D imaging of individual glass particles. The AFM calculated roughness comes as a complementary and local result to characterise the surfaces. AFM gives a higher lateral resolution (<30 nm) compared to optical profilometry (2 ␮m) and a smaller surface size of investigation (20 ␮m × 20 ␮m for AFM and 1000 ␮m × 1000 ␮m for profilometry). Hence, AFM roughness is representative of a local order rather than a global roughness provided by the profilometry. Indeed, only limited areas (20 ␮m × 20 ␮m, 10 ␮m × 10 ␮m and 5 ␮m × 5 ␮m) were analyzed by AFM technique and could neither represent a whole specimen surface nor give the average surface roughness of the material [22]. Moreover, filler size distribution might not be homogeneous and AFM views could not be representative depending on the observed area. Although the average roughness (Ra) is usually used to express the surface roughness [32–34], the Ra parameter does not fully describe the surface of the material [45,46]. The other amplitude or spacing parameters should also be evaluated for these composites as well as related to bacterial adhesion, optical features [25,39] or further properties [45]. Surface roughness is a function of the microrelief of the surface created during finishing and polishing [21]. During these processes, abrasion of resin matrix and filler particles can be accompanied: (i) by the softening of matrix due to the production of highly localized heat [47]; (ii) by the creation of residual defects and surface flows caused by dislodgement or debonding of glass fillers [15,17,30,47]; (iii) by scratch lines left by abrasives of greater size [15,35,48]. The microrelief of the surface, especially voids, cracks and pits, is of critical clinical relevance as it has been reported to create protected sites for bacteria [29]. Thus, the numerous large voids resulting from the plucking out of the voluminous fillers at the surface of Grandio observed by SEM (Fig. 4C) contribute most to the greatest roughness of this composite. In other studies the nanofilled hybrid composites containing large glass fillers (1–1.5 ␮m) have also presented voids and craters at their surface after they have been polished [14,21]. Surprisingly, only very few plucked out particles from the PPF were observed at the Synergy D6 surface. The high degree of conversion of the PPF, obtained by thermopolymerization, might explain an improvement in filler bonding quality [49]. The use of glass filler of the smallest size could further explain these observations. Hence, the second best result after polishing in terms of roughness was attributed to Synergy D6, based on assessment by the three different methods of analysis. According to SEM observations, Filtek Supreme exhibited the smoothest surface with only very few voids which were also seen at the 3D height AFM images (Fig. 4B). Their size (range of the cluster fillers) would suggest that, contrary to claims of the manufacturer, the dislodgement

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of entire clusters is possible. However, such voids were rare and were localized in the scratch lines left by coarser abrasives discs. Taken together, AFM and SEM results validated the second hypothesis suggesting that the surface texture is related to the surface morphology. Aluminum oxide-based abrasives devices have already proved their efficiency for composite finishing and polishing procedures [26,32,38,39,41,50]. However, development of new types of dental composites involves the appearance of adapted polishers. CompoSystem discs by Komet were chosen as a newly developed polishing system, which contains the same abrasive as Sof-Lex. The main differences are a honeycomb dispersion of abrasives, which is claimed to provide an efficient substance removal during polishing, and a mandrel that is completely covered by the disk. Nevertheless, the results of this investigation showed that Sof-Lex discs were significantly more efficient than CompoSystem discs in terms of roughness. Hence, the third hypothesis was rejected. Previous studies have also shown that the Sof-Lex discs were creating a smoother surface compared to Super-Snap [25] or Hawe Neos [51] Al2 O3 polishing discs. The effectiveness of the polishing device is related to particle hardness, size and shape of abrasive as well as to physical properties of backing or bonding material [21,25,30]. Since the polishing devices investigated were not fundamentally different (flexible discs, coated with an alumina of nearly the same grit size), the difference in Ra values obtained could be explained by the quantity of abrasives used in the instrument which is lower for the CompoSystem. According to a previous study, the load of the finishing device to the surface influences the polishing result [22]. However, it was also reported that the pressure applied by the disk seemed to be less critical for flexible discs like Sof-Lex [22]. In the present study, a single operator performed finishing procedures in order to better simulate clinical conditions [13]. For the same purpose, immediate polishing was preferred as compared to delayed polishing [27] as no negative effect on surface roughness was noted [50]. Finally, specimens were polished without water spray according to a previous study showing that dry polishing of composites was superior or equal to wet polishing when flexible aluminum oxide abrasive discs were used [52].

5.

Conclusions

According to this study, it can be concluded that using nanoparticles in the resin composite formulation is not sufficient to improve their surface texture after polishing. The average roughness was related to the average glass filler size. The nanofilled composite Filtek Supreme, which contains only nanofillers, showed the best results when associated to SofLex polishing discs.

Acknowledgements The authors would like to thank R. Lucier for her help with the manuscript and J-H Lignot for his technical help.

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