Polymerization efficiency and flexural strength of low-stress restorative composites

d e n t a l m a t e r i a l s 3 0 ( 2 0 1 4 ) 688–694

Available online at www.sciencedirect.com

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Polymerization efficiency and flexural strength of low-stress restorative composites Cecilia Goracci a,∗ , Milena Cadenaro b , Luca Fontanive b , Giuseppe Giangrosso a, Jelena Juloski a,c, Alessandro Vichi a, Marco Ferrari a a b c

Department of Medical Biotechnologies, University of Siena, Siena, Italy Department of Medical Sciences, University of Trieste, Trieste, Italy Clinic for Pediatric and Preventive Dentistry, University of Belgrade, Belgrade, Serbia

a r t i c l e

i n f o

a b s t r a c t

Article history:

Objectives. To assess depth of cure (DOC), degree of conversion (DC), and flexural strength

Received 25 August 2013

(FS) of several resin composites with low-stress behavior.

Received in revised form

Methods. SonicFill (Kerr), SureFil® SDRTM (Dentsply), everX Posterior (GC), Kalore (GC), and

31 October 2013

Filtek Silorane (3M ESPE) were tested. DOC was measured with the Acetone Shake test. DC

Accepted 10 March 2014

was assessed with Fourier Transform Infra-Red spectroscopy on top and at the bottom of 4 mm-thick disk-shaped specimens. Bottom to top ratios of DC percentages were calculated. FS was evaluated with the Three-Point Bending test. DOC, DC, and FS data were statistically

Keywords:

analyzed.

Bulk-fill

Results. SureFil® SDRTM and everX Posterior achieved significantly greater DOC than Kalore

Flexural strength

and Filtek Silorane. Also, SonicFill had significantly greater DOC than Filtek Silorane. Mean

Low-stress

top DCs ranged between 71.46% and 52.44%. Kalore and everX Posterior had significantly

Polymerization

lower top DCs than the other materials. Mean DC values at 4 mm ranged largely from 57.95%

Resin composite

to 6.82%. Kalore and Filtek Silorane had the lowest values of bottom DC and the difference was statistically significant. EverX Posterior and SonicFill recorded significantly higher FSs than the other materials. Significance. SureFil® SDRTM and everX Posterior exhibited DOC over 4 mm, the maximum thickness recommended for bulk placement, while SonicFill recorded DOC values very close to the 4 mm threshold. SonicFill achieved the highest DC at the irradiated surface, as well as at 4 mm depth. SureFil® SDRTM demonstrated similarly uniform curing through the bulk increment. All the tested composites complied with the requirements of FS established by ISO 4049/2009. © 2014 Academy of Dental Materials. Published by Elsevier Ltd. All rights reserved.

∗ Corresponding author at: Department of Medical Biotechnologies, University of Siena, Policlinico Le Scotte, Viale Bracci, 53100 Siena, Italy. Tel.: +39 0577233131; fax: +39 0577233117. E-mail addresses: [email protected], [email protected] (C. Goracci).

http://dx.doi.org/10.1016/j.dental.2014.03.006 0109-5641/© 2014 Academy of Dental Materials. Published by Elsevier Ltd. All rights reserved.

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1.

Introduction

Polymerization shrinkage stress of resin-based composites can affect marginal integrity and lead to marginal leakage, debonding, secondary caries, post-operative sensitivity, development of perimarginal white lines [1–6]. Curing stress can also be responsible for cusp deflection in high C-factor direct composite restorations, such as large Class I and mesiooccluso-distal Class II cavities [7]. Since all the mentioned conditions adverse the durability of resin-based restorations, research has constantly aimed at the development of materials with low-stress behavior and recently pursued the introduction of novel composites for ‘bulk’ application. Owing to innovations in monomer chemistry, filler characteristics or polymerization kinetics, such materials provide low curing shrinkage that may enable the omission of incremental layering, thus easing the restorative procedure and saving chair time [6]. However, for effective bulk-filling of large and deep cavities, other characteristics of the restorative composites are desirable beside low shrinkage. Particularly, optical properties and photoiniating system should ensure adequate depth of cure to the bulk-applied increment. Several recently marketed ‘bulk-fill’ materials have been claimed to achieve adequate polymerization through a depth of over 4 mm [8–12]. In order to assess the maximal increment thickness of resin composites, researchers have referred to depth of cure (DOC) measurements recorded according to ISO 4049:2000 and ISO 4049:2009 [13–17]. Polymerization efficiency of resin composites has also been assessed by measuring the degree of conversion (DC) with spectroscopic techniques that infer the amount of remaining double bonds [3,18]. Current literature provides DOC and DC data for several materials for bulk filling [3,15–17,19–22]. However, in most of the published studies DC was not measured at the clinically relevant depth for bulkfill composites of 4 mm [20,21]. Moreover, DOC was assessed as per ISO 4049 with the ‘Scrape test’ [15–17], yet the suitability of this method for bulk-fill composites has recently been criticized for providing an overestimation of curing depth in comparison with Vickers hardness profiles [16]. In general, the procedure of scraping off the uncured resin-based material has been considered difficult to standardize [23,24], and the Acetone Shake test, a method involving physical removal of the unreacted monomers, has been preferred by some researchers [24–26]. Another clinically relevant feature of a bulk-fill composite is the ability to function as a ‘dentin-replacement’ material. Such potential can be estimated in laboratory research through the assessment of mechanical properties. Mechanical properties can be expected to vary quite largely among the available bulk-fill composites, in relation to differences in filler load and characteristics. Some products present flowable consistency to enable self-adaptation to cavity walls [8,11], while other materials have higher filler load [10], or feature short glass fibers for reinforcement [12,15]. Among the macromechanical properties that can be tested in laboratory, flexural strength provides an estimate of the composite resin potential to serve as dentin-replacement in high stress bearing areas. The present study was conducted to assess DOC with the Acetone Shake test and DC at a clinically relevant depth

for several resin composites with low-stress behavior. A further objective of the investigation was to measure the flexural strength (FS) attained by the same materials. The null hypotheses that the materials achieve similarly efficient cure and comparable FS were placed under test.

2.

Materials and methods

The following resin composites for bulk-filling of posterior restorations were tested: SonicFill (Kerr, Orange, CA, USA), SureFil® SDRTM (Dentsply, Milford, DE, USA), everX Posterior (GC, Tokyo, Japan). Additionally, the nanohybrid composite Kalore (GC, Tokyo, Japan) and the silorane-based composite Filtek Silorane (3M ESPE, St. Paul, MN, USA), exhibiting lowstress behavior, although not specifically marketed for bulk placement [27,28], were included in the study in order to verify their applicability in this simplified filling technique. The chemical composition of the materials is reported in Table 1.

Table 1 – Chemical composition of the tested materials. Material Filtek Silorane (3M ESPE, St. Paul, MN, USA)

Kalore (GC Corp., Tokyo, Japan)

SonicFill (Kerr Corp., Orange, CA, USA)

SureFil® SDRTM (Dentsply De Trey, Konstanz, Germany)

everX Posterior (GC, Tokyo, Japan)

Chemical composition Silorane resin; quartz filler; yttrium fluoride; initiating system: camphorquinone, iodonium salt, electron donor; stabilizers; pigments. Filler load 76 wt%; 55 vol% Urethane dimethacrylate, DX-511 co-monomers, dimethacrylate (18 wt%); pre-polymerized filler (20–30 wt%); fluoroaluminosilicate glass (20–33 wt%); strontium/barium glass (20–33 wt%); silicon dioxide nanofiller (1–5 wt%); camphorquinone (<1 wt%), pigment (<1 wt%). Filler load 82 wt%; 69 vol% Glass, oxide, chemicals (10–30%); 3-trimethoxysilylpropyl methacrylate (10–30%); silicon dioxide (5–10%); ethoxylated bisphenol-A-dimethacrylate (1–5%); bisphenol-A-bis-(2-hydroxy-3mehacryloxypropyl) ether (1–5%); triethylene glycol dimethacrylate (1–5%). Filler load 83.5 wt%; 83 vol% SDRTM patented urethane dimethacrylate resin, ethoxylated bisphenol A dimethacrylate, triethylene glycol dimethacrylate, butylated hydroxyl toluene, barium-alumino-fluoro-borosilicate glass, strontium alumino-fluoro-silicate glass, camphorquinone, UV stabilizer, titanium dioxide, iron oxide pigments. Filler load 68 wt%; 44%vol% Bisphenol A-diglycidyl dimethacrylate, triethylene glycol dimethacrylate (24.4 wt%); polymethyl methacrylate (0.9 wt%); E-glass fibers, barium borosilicate glass filler (74.2 wt%); camphorquinone, 2-dimethylamino ethyl dimethacrylate, hydroquinone (0.5 wt%). Filler load 74.2 wt%; 53.6 vol%

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For the materials available in more shades, in accordance with ISO 4049/2009, only shade A3 was used, in order to minimize the effects of shade on light polymerization [14].

peak (Eqs. (1) and (2) for methacrylates and silorane respectively) DC% = 1 −

2.1.

Degree of conversion (DC)

DC was determined using a Fourier Transform Infra-Red Attenuated Total Reflectance equipment (FTIR-ATR, Nicolet 6700, Thermo scientific, Milan, Italy). A silicon mold was used to prepare disk-shaped specimens (height = 4 mm, diameter = 4 mm) of each composite (n = 5), which were placed and polymerized in bulk. During photopolymerization a mylar strip was positioned between the curing tip and the specimen to exclude oxygen. Photopolymerization was performed for 20 s, using the same light-curing unit as in the preparation of specimens for DOC measurements, and applying the curing tip on the top of the specimen. The specimens were then placed on the diamond crystal of a horizontal attenuated total reflectance (ATR) stage of the FTIR equipment. Infrared (IR) spectra were obtained between 4000 and 500 cm−1 at a resolution of 6 cm−1 on top and at the bottom of each specimen immediately after photopolymerization. For the methacrylates-based composites, the FTIR bands used to calculate the DC were the carbonyl (C O) peak at 1715 cm−1 as an internal reference and the vinyl C C peak at 1635 cm−1 as a reaction peak. For the silorane composite the signals used to determine the DC were the stretching vibration of the epoxy ring (C O C) at 884 cm−1 as a reaction peak and of the Si CH3 bond at 695 cm−1 as a reference peak. The DC was calculated using the ratio between the reaction and internal reference

(t=n)

/(C = O)

(t=n)

)

(t=0)

/(C = O)

(t=0)

)

((C = C)

Depth of cure (DOC)

DOC of the resin composite was assessed with the Acetone Shake test [24–26]. Each material was placed in bulk in a cylindrical silicone mold (height 12 mm, internal diameter 5 mm). A mylar strip was then pressed over the applied material with a glass plate to obtain a flat surface. Following manufacturers’ recommendations, light-curing was performed for 20 s using Demi LED light (Orange, CA, USA; output 1100 mW/cm2 ; tip diameter 8 mm), placing the light tip on the exposed side of the specimen. After curing and removal from the mold, the resin composite cylinder was placed into a hermetically sealed capsule containing 99.9 per cent pure acetone (2004/000113, Polichimica srl, Bologna, Italy). The capsule was vibrated on a mixing device for 15 s (TAC-135/A n◦ 3437, Tac Dental, Asti, Italy) [25,26]. Such vibrating motion removed the uncured material in a reproducible manner [24], leaving the polymerized portion undamaged. At this stage, the resin composite specimen was removed from the container and dried. The height of the specimen was measured in mm using a digital caliper with a 10 ␮m resolution (Mitutoyo, Miyazaki, Japan) and divided by two, as defined for the ‘Scrape test’ in ISO 4049. The rationale for such subdivision is that not all the hardened specimen is in fact optimally polymerized [16], and dividing the remaining sample thickness by two has been considered to provide a ‘safe’ measure of adequate cure.

2.2.

((C = C)

(t=n)

DC% = 1 −

((C–O–C)

(t=0)

((C–O–C)

× 100

/(Si–CH3 )

(t=n)

)

/(Si–CH3 )

(t=0)

)

× 100

(1)

(2)

where n = 20 s.

2.3.

Flexural strength (FS)

A vinylpolysiloxane-based mold (Elite H-D+, Zhermack Spa, Badia Polesine, Italy) 25 mm in length, 2.1 mm in height, 2.1 mm in width was bulk-filled with the resin composite. In accordance with ISO 4049/2009 [14], the bulk increment was light-cured first on the exposed side of the specimen with three light irradiations of 20 s, overlapping by half of the diameter of the curing light tip [3,22]. Then, the specimen was removed from the mold and the curing procedure was repeated on the opposite side. The obtained composite bar was wet-ground with 600 and 1200 FEPA metallographic paper, until the dimensions of 2.0 ± 0.1 mm in height and 2.0 ± 0.1 mm in width were obtained, as requested by the ISO Standard 4049/2009 [14]. After a 24-hour storage in distilled water at 37 ◦ C [3], the specimens were subjected to ThreePoint Bending test, using a device designed in accordance with the ISO Standard 4049/2009 [14]. The appliance was made of two HSS/Cobalt rods (diameter 2 mm) mounted parallel with 20 mm between centers (support span 20 mm). Each specimen was loaded at its center with a cylindrical-ended striker (diameter 2 mm). A crosshead speed of 0.75 mm/min was applied in the universal testing machine until failure occurred. FS was calculated in MegaPascals (MPa) with the following equation:

=

3Fl 2bh2

where  is FS, F is the maximum load (N), l is the distance between the supports (mm), b is the specimen width (mm) and h is the specimen height (mm).

2.4.

Statistical analysis of DOC data

As the data distribution was not normal according to the Kolmogorov–Smirnov test, the Kruskal–Wallis analysis was applied to assess the statistical significance of the betweengroup differences in DOC. Subsequently, the Dunn’s Multiple Range test was used for post hoc comparisons. In all the analyses the level of significance was set at p < 0.05.

2.5.

Statistical analysis of DC data

Two separate One-Way Analyses of Variance (ANOVA) were applied to DC values measured on top and at the bottom of the specimens, having verified that each data set met the requirements of normality of data distribution and homogeneity of group variances (Kolmogorov–Smirnov test and Levene test, respectively). The Tukey test was applied for post

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Table 2 – Descriptive statistics of depth of cure (DOC) in mm. In the significance column different letters label statistically significant between-group differences. Group

N

Mean

SureFil® SDRTM everX Posterior SonicFill Kalore Filtek Silorane

10 10 10 10 10

5.35 5.29 3.47 3.32 2.23

Standard deviation 0.37 0.19 0.03 0.08 0.04

Median

Interquartile range

5.36 5.25 3.47 3.33 2.25

5.03–5.68 5.11–5.41 3.44–3.48 3.25–3.37 2.22–2.26

Significance (p < 0.05) A A AB BC C

hoc comparisons. The same tests were also used to assess the significance of between-group differences in bottom to top ratio of DC values. In all the analyses the level of significance was set at p < 0.05.

and Filtek Silorane (p < 0.05). Also, SonicFill had significantly greater DOC than Filtek Silorane (p < 0.05).

2.6.

Descriptive statistics of top DC values, bottom DC values, and bottom to top ratios are reported in Tables 3–5, respectively. Mean top DCs ranged between 71.46% and 52.44%. Kalore and everX Posterior had significantly lower top DCs than the other materials (p < 0.05). Mean DC values at 4 mm ranged largely from 57.95% to 6.82%. Kalore and Filtek Silorane had the lowest values of bottom DC and the difference was statistically significant (p < 0.05). Progressively higher DCs were measured respectively for everX Posterior, SureFil® SDRTM , and SonicFill, with statistically significant between-group differences (p > 0.05). SonicFill exhibited the highest bottom to top DC ratio (0.81 ± 0.07). Only SureFil® SDRTM had a statistically comparable ratio. All the other between-group differences were statistically significant.

Statistical analysis of FS data

Having preliminarily checked that data distribution was normal in each group and that group variances were homogeneous (Kolmogorov–Smirnov test and Levene test, respectively), the One-Way ANOVA was applied to verify the existence of statistically significant between-group differences, followed by the Tukey test for post hoc comparisons. In all the analyses the level of significance was set at p < 0.05.

3.

Results

3.1.

Depth of cure (DOC)

Descriptive statistics of DOC values are reported in Table 2. DOC ranged between 5.35 ± 0.37 mm of SureFil® SDRTM to 2.23 ± 0.04 mm of Filtek Silorane. The Kruskal–Wallis ANOVA revealed that groups differed significantly (p < 0.001). Specifically, according to the post hoc test, SureFil® SDRTM and everX Posterior achieved significantly higher DOC than Kalore

3.2.

3.3.

Degree of conversion (DC)

Flexural strength (FS)

FS data are summarized in Table 6. The One-Way ANOVA indicated that groups differed significantly (p < 0.001). Specifically, the multiple comparison test pointed out that everX Posterior and SonicFill recorded the highest FSs that did not differ

Table 3 – Descriptive statistics of degree of conversion (DC) percentages measured on top of the resin composite specimen. In the significance column different letters label statistically significant between-group differences. Group

N

Mean

Standard deviation

SonicFill Filtek Silorane SureFil® SDRTM everX Posterior Kalore

5 5 5 5 5

71.46 70.75 70.18 54.79 52.44

2.15 4.96 2.01 4.66 3.22

Significance (p < 0.05) A A A B B

Table 4 – Descriptive statistics of degree of conversion (DC) percentages measured at the bottom of the resin composite specimen (4 mm depth). In the significance column different letters label statistically significant between-group differences. Group

N

Mean

SonicFill SureFil® SDRTM everX Posterior Kalore Filtek Silorane

5 5 5 5 5

57.95 50.36 37.95 7.63 6.82

Standard deviation 4.77 2.34 2.01 4.32 4.43

Significance (p < 0.05) A B C D D

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Table 5 – Descriptive statistics of bottom to top ratios of degree of conversion (DC) percentages. In the significance column different letters label statistically significant between-group differences. Group

N

Mean

Standard deviation

SonicFill SureFil® SDRTM everX Posterior Kalore Filtek Silorane

5 5 5 5 5

0.81 0.72 0.69 0.14 0.09

0.07 0.04 0.04 0.07 0.06

Significance (p < 0.05) A AB B C C

Table 6 – Descriptive statistics of flexural strength (FS) measurements in MPa. In the significance column different letters label statistically significant between-group differences. Group

N

Mean

Standard deviation

everX Posterior SonicFill SureFil® SDRTM Kalore Filtek Silorane

10 10 10 10 10

201.08 193.45 137.26 114.66 112.64

37.15 38.64 18.15 7.26 28.64

between each other. Significantly lower values were achieved by SureFil® SDRTM , Filtek Silorane, and Kalore.

4.

Discussion

As the statistical analyses revealed the existence of statistically significant differences among the tested composites with regard to DOC, DC, and FS, the formulated null hypothesis had to be rejected. In the present study DOC was assessed as it represents a critical property for bulk-fill composites [23]. The ISO 4049/2009 states that DOC values of resin-based restorative materials should be ‘no more than 0.5 mm below those stated by the manufacturer’ [14]. Manufacturers of bulk-fill composites have claimed that their materials can be cured in a thickness of at least 4 mm [8–12]. Therefore, the data collected in the present study showed that SureFil® SDRTM and everX Posterior met ISO 4049/2009 requirement for DOC, while SonicFill DOC was very closed to the threshold value. Kalore and Filtek Silorane had relatively lower DOC. Therefore, even though the low-stress behavior of these composites might suggest a potential application in the bulk-filling technique, the concern that the materials may suffer reduced polymerization at depth discourages such use of Kalore and Filtek Silorane. The findings of DOC measurements were confirmed by the occurrence of very low DC percentages at 4 mm depth for Kalore and Filtek Silorane. It is however worth reiterating that manufacturers indeed recommend incremental placement for these materials [27,28]. Particularly, for Filtek Silorane a maximum increment thickness of 2.5 mm has been advised [28]. Using the ‘Scrape’ method, Kusgoz et al. measured DOC values of 4.9 ± 0.1 mm and 4.8 ± 0.2 mm respectively, when Filtek Silorane was cured for 40 s with a quartz-tungsten-halogen light and for 20 s with an LED light [17]. Still with the ‘Scraping test’, Flury et al. [16] reported median DOC values as low as 1.76 mm and 2.09 mm for Filtek Silorane respectively after 10-s and 20-s polymerization with an LED light. Kusgoz et al. [17] speculated that the peculiar initiating system, composed of camphorquinone, an iodonium salt, and an electric donor,

Significance (p < 0.05) A A B B B

may be responsible for the lower DOC demonstrated by Filtek Silorane, in comparison with methacrylate-based composites. For the nanohybrid composite Kalore the manufacturer, using the ‘Scrape test’, reported 2.8 mm of DOC for A2 shade [27]. This measurement is very close to the mean value recorded for A3 shade with the Acetone Shake test in the present study. The curing efficiency of SureFil® SDRTM was found to be overall satisfactory and this result is in line with the findings of previous investigations [3,19,20]. The peculiar photoinitiating system may have contributed to such outcome. SureFil® SDRTM features a photoactive group embedded in urethanebased methacrylate monomers and capable of interacting with camphorquinone [8]. Such interaction, claimed to modulate curing for stress control purposes, might also have resulted in deeper polymerization. Beside the photoinitiating system, also the optical properties might have had a role in this regard. Specifically, the translucency of SureFil® SDRTM is expected to favor light penetration, thus enabling increased DOC. As a matter of fact the light transmitting ability of resin composites has been found to relevantly affect polymerization depth [29]. SonicFill demonstrated high bottom to top ratio of DC percentages, which provided an indication of uniform curing through the thickness of the bulk increment. Although the influence of ultrasonic energy on resin curing has not yet been systematically assessed, it can be speculated that the thermal effect of sonic vibration may promote polymerization by increasing free radicals mobility directly and indirectly as a consequence of decreased viscosity. SonicFill recorded also the highest DC at the irradiated surface. For SonicFill, as well as for Filtek Silorane and SureFil® SDRTM , the top DC was above the threshold of 55% requested to have adequate wear resistance at the occlusal level of the restoration [20]. For everX Posterior DOC in excess of 4 mm, suitable for the use in bulk-filling technique, was noted. This finding is in line with the result of the ‘Scrape test’ performed in the study by Garoushi et al. [15]. Conversely, DOC measurements did not correspond well with DC percentages recorded at 4 mm depth in the present investigation.

d e n t a l m a t e r i a l s 3 0 ( 2 0 1 4 ) 688–694

FS has been assessed in several previous studies as a clinically relevant property for restorative materials meant to be used in high-stress bearing areas [15,19,21,22,30–34]. All the tested composites demonstrated FS over the limit of 80 MPa established in ISO 4049/2009 ‘for polymer-based restorative materials claimed by the manufacturer as suitable for restorations involving occlusal surfaces’ [14]. The highest FSs were measured by the fiber-reinforced resin composite everX Posterior. This observation agreed with the result of the study by Garoushi et al. [15]. According to these Authors, everX Posterior fibers, being longer than the critical fiber length of E-glass with Bis-GMA polymer matrix, allow for stress transmission from matrix to fibers, thus producing an effective reinforcement. The material is also purportedly strengthened as a result of the ability of the randomly oriented fibers to function as crack stoppers [15]. Additionally, the so-called ‘semi-Interpenetrating Polymer Network’, a resin matrix containing bis-GMA, TEGDMA, and PMMA, would increase the fracture toughness of the composite resin [35]. It should however be pointed out that everX Posterior is not suitable for occlusal surfaces, and the manufacturer recommends to cover it with a layer of a restorative composite with adequate wear resistance and surface gloss [12]. It therefore seems of interest to verify to what extent the layering of a veneering composite over base filling may affect the load bearing capacity. As a matter of fact, the other material that achieved comparably high FS, SonicFill, does not require any additional layer of composite, thus enabling to perform a single-step restoration. The high filler load of SonicFill purportedly renders the composite capable of sustaining the functional stress, while sonic energy lowers the viscosity for improved interfacial adaptation [10]. Nevertheless, the data collected in the present investigation suggest that high filler percentage does not necessarily reflect into superior FS. In fact, the flowable composite SureFil® SDRTM , with a filler load of 68% by weight, 44% by volume, obtained higher FS than the more heavily filled Kalore. Also Garoushi et al. [15] noted the absence of a direct relationship between volumetric content of filler and fracture parameters such as fracture toughness and FS of several commercial composites. These Authors claimed that other factors beside filler content, such as stress transfer between filler particles and matrix, as well as adhesion between these components may play a relevant role [15]. Moreover, recent research has focused the attention on the influence of resin matrix chemistry on the composites mechanical properties [18]. It might be argued that FS was measured on 2-mm thick specimens, while bulk-fill composites are clinically applied in thicker layers. It should however be pointed out that the choice to test 2-mm thick bars was justified by the need to conform to ISO 4049/2009 [14] in performing the FS test, as also done in previous studies [15,22,31].

5.

Conclusions

SureFil® SDRTM and everX Posterior exhibited DOC in excess of 4 mm, the maximum thickness recommended for bulk placement, while SonicFill recorded DOC values very close to the 4-mm threshold. SonicFill also achieved the highest DC at

693

the irradiated surface, as well as at 4 mm depth. SureFil® SDRTM demonstrated similarly uniform curing through the bulk increment. All the tested composites had FS over the 80 MPa established by ISO 4049/2009 as the limit of acceptability for restorative materials reproducing occlusal surfaces. The fiberreinforced and the sonic-activated composites obtained the highest FSs. A direct correspondence between filler load and FS was not found.

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