d e n t a l m a t e r i a l s 3 0 ( 2 0 1 4 ) 1104–1112

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Influence of increment thickness on microhardness and dentin bond strength of bulk fill resin composites Simon Flury ∗ , Anne Peutzfeldt, Adrian Lussi Department of Preventive, Restorative and Pediatric Dentistry, School of Dental Medicine, University of Bern, Switzerland

a r t i c l e

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a b s t r a c t

Article history:

Objectives. To investigate the influence of increment thickness on Vickers microhardness

Received 14 February 2014

(HV) and shear bond strength (SBS) to dentin of a conventional and four bulk fill resin

Received in revised form 2 May 2014

composites.

Accepted 2 July 2014

Methods. HV and SBS were determined on specimens of the conventional resin composite Filtek Supreme XTE (XTE) and the bulk fill resin composites SDR (SDR), Filtek Bulk Fill (FBF), x-tra fil (XFIL), and Tetric EvoCeram Bulk Fill (TEBF) after 24 h storage. HV was mea-

Keywords:

sured either as profiles at depths up to 6 mm or at the bottom of 2 mm/4 mm/6 mm thick

Surface hardness

resin composite specimens. SBS of 2 mm/4 mm/6 mm thick resin composite increments was

Depth of cure

measured to dentin surfaces of extracted human molars treated with the adhesive system

Dentin bonding

OptiBond FL, and the failure mode was stereomicroscopically determined at 40× magnifica-

Light-curing

tion. HV profiles and failure modes were descriptively analysed whereas HV at the bottom of

Posterior restorative

resin composite specimens and SBS were statistically analysed with nonparametric ANOVA followed by Wilcoxon rank sum tests (˛ = 0.05). Results. HV profiles (medians at 2 mm/4 mm/6 mm): XTE 105.6/88.8/38.3, SDR 34.0/35.5/36.9, FBF 36.4/38.7/37.1, XFIL 103.4/103.9/101.9, TEBF 63.5/59.7/51.9. HV at the bottom of resin composite specimens (medians at 2 mm/4 mm/6 mm): XTE (p < 0.0001) 105.5 > 85.5 > 31.1, SDR (p = 0.10) 25.8 = 21.9 = 26.0, FBF (p = 0.16) 26.6 = 25.3 = 28.9, XFIL (p = 0.18) 110.5 = 107.2 = 101.9, TEBF (p < 0.0001) 63.0 > 54.9 > 48.2. SBS (MPa, medians at 2 mm/4 mm/6 mm): XTE (p < 0.0001) 23.9 > 18.9 = 16.7, SDR (p = 0.26) 24.6 = 22.7 = 23.4, FBF (p = 0.11) 21.4 = 20.3 = 22.0, x-tra fil (p = 0.55) 27.0 = 24.0 = 23.6, TEBF (p = 0.11) 21.0 = 20.7 = 19.0. The predominant SBS failure mode was cohesive failure in dentin. Significance. At increasing increment thickness, HV and SBS decreased for the conventional resin composite but generally remained constant for the bulk fill resin composites. © 2014 Academy of Dental Materials. Published by Elsevier Ltd. All rights reserved.



Corresponding author at: Freiburgstrasse 7, CH-3010 Bern, Switzerland. Tel.: +41 316322581; fax: +41 316329875. E-mail address: simon.fl[email protected] (S. Flury) .

http://dx.doi.org/10.1016/j.dental.2014.07.001 0109-5641/© 2014 Academy of Dental Materials. Published by Elsevier Ltd. All rights reserved.

Filler load: 60–61 vol%, 79–81 wt%

Matrix: (dimethacrylates) IVA (reddish universal shade) 4 Packable resin composite for bulk fill

Tetric EvoCeram Bulk Fill (Ivoclar Vivadent, Schaan, Liechtenstein) LOT-number: S09723

Matrix: Bis-GMA, UDMA, TEGDMA Filler load: 70.1 vol%, 86 wt% Universal 4 Packable resin composite for bulk fill x-tra fil (VOCO, Cuxhaven, Germany) LOT-number: 1316512

Matrix: Bis-GMA, UDMA, Bis-EMA(6), procrylat resins Filler load: 42.5 vol%, 64.5 wt% Universal 4 Flowable resin composite for bulk fill Filtek Bulk Fill (3M ESPE, St. Paul, MN, USA) LOT-number: N421893

LOT-number: 120723

Microhardness (Vickers hardness; HV) profiles were generated by means of a re-usable, block-shaped, and custom-made Teflon mold with a semicircular notch of 15 mm in length

Matrix: modified UDMA, EBPADMA, TEGDMA Filler load: 45 vol%, 68 wt% Universal 4 Flowable resin composite for bulk fill SDR (DENTSPLY Caulk, Milford, DE, USA)

Microhardness profiles

Matrix: Bis-GMA, UDMA, TEGDMA, PEGDMA, Bis-EMA Filler load: 63.3 vol%, 78.5 wt% A3 2 Packable conventional resin composite Filtek Supreme XTE (3M ESPE, St. Paul, MN, USA) LOT-number: N470314

Five resin composites (Table 1) were used in the present study: one packable conventional resin composite as a control (Filtek Supreme XTE), two flowable resin composites for bulk fill (SDR and Filtek Bulk Fill), and two packable resin composites for bulk fill (x-tra fil and Tetric EvoCeram Bulk Fill). All lightcuring was performed with an LED light-curing unit (Demi, Kerr Corporation, Middleton, WI, USA) and light power density was verified to be at least 1000 mW/cm2 at the beginning and end of each day of specimen preparation with a radiometer (Demetron L.E.D. Radiometer, Kerr Corporation).

2.1.

Maximum increment thickness (mm) (according to manufacturer)

Materials and methods

Table 1 – Resin composites used.

2.

Shade

When restoring cavities, conventional light-curing resin composites should be placed in increments of a thickness generally not exceeding 2 mm [1–3]. Consequently, when restoring deep cavities, placement and light-curing of conventional resin composites in numerous increments of 2 mm thickness are needed. As such a procedure is rather timeconsuming, new types of light-curing resin composites with an increased maximum increment thickness have been marketed, the so-called “bulk fill” resin composites. Generally, bulk fill resin composites are claimed to be curable to a thickness of 4 mm [4–7] resulting in a need for fewer increments and thus, in an economy of time. In a previous study, microhardness of both conventional and bulk fill resin composites was measured at increasing distances from the irradiated surface [8]. The resulting surface hardness profiles showed a gradual decrease in microhardness from the “top” toward the “bottom” and this decrease markedly varied depending on the type of resin composite [8]. Since microhardness measurement has been deemed a useful method to indirectly probe polymer network conversion [9–11], the gradual decrease in microhardness along the surface hardness profiles suggests a decrease in the degree of conversion of the resin composites with increasing distance from the irradiated surface. A decrease in the degree of conversion as well as an increase in increment thickness have been shown to negatively affect bond strength of resin composites to dentin [12,13]. Consequently it is of paramount importance for the longevity of the restorations that light-curing is fully adequate at the bottom surface of each increment of resin composite. Thus, the aim of the present study was to investigate the influence of increment thickness on microhardness and on shear bond strength to dentin of bulk fill resin composites and to compare with a conventional resin composite. The study tested the following working hypotheses: (1) microhardness decreases with increasing increment thickness of the resin composites and (2) shear bond strength to dentin decreases with increasing increment thickness of the resin composites.

Type

Matrix and filler load (according to manufacturer)

Introduction

Resin composite

1.

Bis-GMA, Bisphenol A glycidyl dimethacrylate; UDMA, urethane dimethacrylate; TEGDMA, triethyleneglycol dimethacrylate; PEGDMA, poly(ethylene glycol) dimethacrylate; Bis-EMA, Bisphenol A polyethylene glycol diether dimethacrylate; EBPADMA, ethoxylated Bisphenol A dimethacrylate.

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d e n t a l m a t e r i a l s 3 0 ( 2 0 1 4 ) 1104–1112

Fig. 1 – Specimen preparation. Microhardness profiles: A, Teflon mold with semicircular notch; B, Teflon shell; C, semicircular opening for light-curing; D, mold including the light-cured resin composite specimen. Microhardness (HV) and shear bond strength (SBS) at increasing increment thickness: E, split Teflon molds; F, dentin underlay (mid-coronal dentin of a ground, embedded human molar); G, mold filled with resin composite for future HV measurement; H, dentin specimen (mid-coronal dentin of ground, embedded human molars); I, specimen for future SBS measurement.

and 4 mm in diameter (Fig. 1A). The semicircular notch was entirely filled with one of the five resin composites. Then, the mold was covered with a Mylar strip (Hawe Stopstrip Straight, KerrHawe, Bioggio, Switzerland) and the resin composite in the semicircular notch was made flush with the mold by use of a glass slide. Excessive resin was removed and the mold was covered by a Teflon shell (Fig. 1B). A second Mylar strip was placed on the semicircular opening (Fig. 1C) and the resin composite was light-cured for 20 s through the semicircular opening (top surface) keeping the tip end of the light-curing unit centered and in contact with the second Mylar strip. After light-curing, the shell and both Mylar strips were removed and the mold including the resin composite specimen (Fig. 1D) was placed in a black photo-resistant box at 100% humidity and the box was stored in an incubator (Memmert UM 500, Memmert GmbH & Co., Schwabach, Germany) at 37 ◦ C for 24 h. After storage, the mold including the resin composite specimen was placed under a microhardness indentation device (Fischerscope HM2000, Helmut Fischer GmbH, Sindelfingen, Germany). Subsequently, six HV measurements were made on the resin composite specimen at defined distances (“depths”), beginning with an HV measurement at a depth of 1 mm from the irradiated top surface followed by HV measurements at a depth of 2 mm, 3 mm, 4 mm, 5 mm, and 6 mm (i.e. at increasing

distances from the top surface). Programming of the hardness indentation device for defined distances and reproducible placement of the mold ensured that the HV measurements were made along the same axis on each specimen. HV measurements were performed in a force-controlled mode for 50 s with the test load increasing and decreasing between 0.4 and 500 mN at a constant speed. For each of the five resin composites, n = 8 specimens were prepared resulting in eight HV profiles per resin composite.

2.2.

Microhardness at increasing increment thickness

Another 210 resin composite specimens were produced for measurement of microhardness (Vickers hardness; HV) on the bottom surface (i.e. on the surface opposing the irradiated top surface) of specimens of increasing increment thickness (n = 14/group; 15 groups (5 resin composites, 3 increment thicknesses)). Specimens of the five resin composites were prepared in re-usable custom-made Teflon molds (split Teflon molds with an inner diameter of 3.6 mm and a height of 2 mm, 4 mm, or 6 mm mimicking three increment thicknesses (Fig. 1E)). The molds were the same as those later used for preparation of shear bond strength specimens. During preparation of HV specimens, the molds were placed on a

d e n t a l m a t e r i a l s 3 0 ( 2 0 1 4 ) 1104–1112

dentin surface used as underlay in analogy to preparation of specimens bonded to dentin for measurement of shear bond strength. In order to obtain the dentin surface, one extracted human molar was ground from the occlusal surface until at mid-coronal dentin. Grinding was performed with a grit #220 SiC abrasive paper (Struers, Ballerup, Denmark) on a grinding machine (Struers LaboPol-21, Struers). The molar was embedded in self-curing acrylic resin (Paladur, Heraeus Kulzer GmbH, Hanau, Germany) in a cylindrical stainless steel mold. After removal of the mold, the mid-coronal dentin of the embedded molar was ground with a grit #500 SiC abrasive paper (Struers). Centrally, the dentin surface of the embedded molar exhibited a shade of A4/5M3 (VITA Easyshade Advance 4.0, VITA Zahnfabrik, Bad Säckingen, Germany). The dentin underlay (Fig. 1F) was re-used throughout the study and when not in use, it was stored at 100% humidity and 37 ◦ C. During preparation of the HV specimens, the dentin underlay was covered with a first Mylar strip (Hawe Stopstrip Straight, KerrHawe) prior to placement of the respective Teflon mold. The mold was then filled in bulk with one of the five resin composites and covered with a Mylar strip. The resin composite was made flush with the mold by use of a glass slide and after light-curing for 20 s, the filled mold was removed from the dentin underlay and from both Mylar strips (Fig. 1G). The mold was then placed in a black photo-resistant box at 100% humidity and the box was stored in an incubator (Memmert UM 500, Memmert GmbH & Co.) at 37 ◦ C for 24 h. After storage, HV measurements were performed on the bottom surface of each specimen (i.e. the surface formerly in contact with the first Mylar strip and opposing the irradiated top surface). Five HV measurements per specimen were performed with the same microhardness indentation device and in the same force-controlled mode as described above (one HV measurement in the center and four HV measurements toward the periphery of the specimen). Out of the five HV measurements per specimen, one mean value was calculated resulting in 14 HV values per group for statistical analyses.

2.3. Shear bond strength at increasing increment thickness A final amount of 255 extracted human molars without restorations or caries was used for measurement of shear bond strength (SBS) at increasing increment thickness (n = 17 molars/group; 15 groups (5 resin composites, 3 increment thicknesses)). The molars were cleaned under tap water with a scaler and a hard toothbrush to remove calculus and debris, stored in 2% chloramine solution, and kept at 4 ◦ C until needed. For preparation of dentin specimens (Fig. 1H), molars were apically shortened with a water-cooled diamond saw (IsoMet Low Speed Saw, Buehler, Lake Bluff, IL, USA) and ground from the occlusal surface until at mid-coronal dentin. Grinding was performed with a grit #220 SiC abrasive paper (Struers) on a grinding machine (Struers LaboPol-21, Struers). The molars were embedded in Paladur (Heraeus Kulzer GmbH) in cylindrical stainless steel molds. After removal of the molds, the mid-coronal dentin of the embedded molars was ground with grit #500 SiC abrasive papers (Struers) for 10 s to obtain a standardized smear layer. The grit #500 SiC abrasive papers

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were changed after grinding of 10 dentin specimens. The dentin specimens were subsequently stored in tap water at room temperature. The ground, dentin surfaces of the specimens were air-dried and for each specimen, the bonding area on the dentin surface was defined and isolated by use of perforated self-adhesive tape (diameter of the perforation ∼4 mm). The bonding area of the dentin surface was then treated with the three-step etch-and-rinse adhesive system OptiBond FL (Kerr, Scafati, Italy) according to manufacturer’s instructions (etching (15 s; Gel Etchant; LOT-number: 4841467), water spray (>15 s), air-dry (∼3 s); OptiBond FL Prime (15 s; LOT-number: 4856727), air-dry (∼5 s); OptiBond FL Adhesive (10 s; LOT-number: 4856728), air-dry (∼3 s), light-cure (5 s)). After adhesive treatment, a split Teflon mold as described above (inner diameter: 3.6 mm; height: 2 mm, 4 mm, or 6 mm (Fig. 1E)) was clamped to the dentin surface of each specimen and the respective mold was filled in bulk with one of the five resin composites and covered with a Mylar strip. The resin composite was made flush with the mold by use of a glass slide and after light-curing for 20 s, the SBS specimen was placed in a black photo-resistant box. Five minutes after completion of light-curing and at room temperature, the specimen was freed from the Teflon mold (Fig. 1I). All specimens were stored in the black photo-resistant boxes at 100% humidity and 37 ◦ C for 24 h. After storage, specimens were subjected to SBS testing by use of a wire (stainless steel, diameter 0.6 mm) in a universal testing machine (Zwick Z010, Zwick GmbH & Co., Ulm, Germany) at a crosshead speed of 1 mm/min. The maximum force (Fmax (N)) was recorded (testXpert software V9.0, Zwick GmbH & Co.) and the SBS values (MPa) were calculated (Fmax (N)/bonding area (mm2 ); bonding area = r2 ×  = ∼10.2 mm2 (radius (r) = 1.8 mm)) resulting in 17 SBS values per group for statistical analyses. After SBS testing, the failure mode of each specimen was determined under a stereomicroscope (Leica ZOOM 2000, Leica, Buffalo, NY, USA) at 40× magnification and classified as (1) cohesive failure in dentin, (2) adhesive failure at dentin – adhesive interface, (3) adhesive failure at adhesive – resin composite interface, (4) cohesive failure in resin composite, or (5) mixed failure (combinations of failure modes (1)–(4)).

2.4.

Statistical analyses

HV values obtained through measurement of HV profiles as well as failure modes after SBS testing were analysed descriptively whereas HV values obtained through measurement of HV at increasing increment thickness and SBS values were analysed with nonparametric one-way ANOVA followed by post hoc Wilcoxon rank sum tests and Bonferroni–Holm adjustment for multiple testing. All calculations were performed with R version 2.12.1 (The R Foundation for Statistical Computing, Vienna, Austria; www.R-project.org). For both HV and SBS, data of preliminary tests was statistically analysed with NCSS/PASS 2005 (NCSS, Kaysville, UT, USA) for sample size determination after the level of significance had been set at ˛ = 0.05.

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3.

Results

3.1.

Microhardness profiles

The microhardness (HV) profiles of the five resin composites are shown in Fig. 2. The packable conventional resin composite Filtek Supreme XTE showed HV medians of 105.6 at 2 mm, 88.8 at 4 mm, and 38.3 at 6 mm depth. The two flowable resin composites for bulk fill, SDR and Filtek Bulk Fill, showed HV medians of 34.0 at 2 mm, 35.5 at 4 mm, and 36.9 at 6 mm depth for SDR and of 36.4 at 2 mm, 38.7 at 4 mm, and 37.1 at 6 mm depth for Filtek Bulk Fill. The two packable resin composites for bulk fill, x-tra fil and Tetric EvoCeram Bulk Fill, showed HV medians of 103.4 at 2 mm, 103.9 at 4 mm, and 101.9 at 6 mm depth for x-tra fil and of 63.5 at 2 mm, 59.7 at 4 mm, and 51.9 at 6 mm depth for Tetric EvoCeram Bulk Fill. Regarding HV at increasing depths, Filtek Supreme XTE showed the most drastic decrease along the HV profile and HV dropped below 80% of the maximum HV value (HVmax ) at depths of more than 4 mm (HVmax (median) = 106.8 at 1 mm depth; 80% of HVmax = 85.4). Tetric EvoCeram Bulk Fill showed a certain decrease in HV at increasing depths. However, up to a depth of 6 mm HV did not drop below 80% of HVmax (HVmax (median) = 64.4 at 1 mm depth; 80% of HVmax = 51.5). A slight decrease in HV was found for x-tra fil at 6 mm compared to at 4 mm depth. SDR as well as Filtek Bulk Fill showed no decrease in HV at increasing depths but rather a slight increase in HV at 4 and 6 mm compared to 2 mm depth.

3.2.

Microhardness at increasing increment thickness

Microhardness (HV) obtained through measurement of HV at increasing increment thickness of resin composite is shown in Fig. 3. The nonparametric one-way ANOVA showed a significant effect of increment thickness on HV for Filtek Supreme XTE (p < 0.0001) and Tetric EvoCeram Bulk Fill (p < 0.0001) but no significant effect for SDR (p = 0.10), Filtek Bulk Fill (p = 0.16), or x-tra fil (p = 0.18). At increment thicknesses of 2 mm/4 mm/6 mm, the resin composites yielded the following HV medians: Filtek Supreme XTE 105.5/85.5/31.1, SDR 25.8/21.9/26.0, Filtek Bulk Fill 26.6/25.3/28.9, x-tra fil 110.5/107.2/101.9, and Tetric EvoCeram Bulk Fill 63.0/54.9/48.2.

3.3. Shear bond strength at increasing increment thickness Shear bond strength (SBS) at increasing increment thickness of resin composite is shown in Fig. 4. The nonparametric oneway ANOVA showed a significant effect of increment thickness on SBS for Filtek Supreme XTE (p < 0.0001) but no significant effect for SDR (p = 0.26), Filtek Bulk Fill (p = 0.11), x-tra fil (p = 0.55), or Tetric EvoCeram Bulk Fill (p = 0.11). At increment thicknesses of 2 mm/4 mm/6 mm, the resin composites yielded the following SBS medians (in MPa): Filtek Supreme XTE 23.9/18.9/16.7, SDR 24.6/22.7/23.4, Filtek Bulk Fill 21.4/20.3/22.0, x-tra fil 27.0/24.0/23.6, and Tetric EvoCeram Bulk Fill 21.0/20.7/19.0.

The distribution of failure modes after SBS testing is shown in Table 2. The predominant failure modes for all resin composites were cohesive failure in dentin and mixed failure. Ninety-two percent of the teeth displaying such mixed failures included partial cohesive failure in dentin, meaning that 213 specimens out of the 255 specimens in total showed either total or partial cohesive failure in dentin. The two adhesive failure modes (i.e. adhesive failure at dentin – adhesive interface and adhesive failure at adhesive – resin composite interface) were found only at an increment thickness of 6 mm, whereas cohesive failure in resin composite was found mainly with the two packable resin composites for bulk fill.

4.

Discussion

Whereas the microhardness of the conventional resin composite, in corroboration with previous findings [8,14–18] and as hypothesized, proved highly sensitive toward increment thickness, three of the bulk fill materials proved completely insensitive, leading to rejection of the first hypothesis. Like the conventional resin composite Filtek Supreme XTE, the fourth bulk fill material (Tetric EvoCeram Bulk Fill) showed a significant decrease in microhardness at increasing increment thickness, but the decrease was much less dramatic than that of Filtek Supreme XTE. In agreement with a previous study [18], the two flowable bulk fill materials (SDR, filler volume = 45%; Filtek Bulk Fill, filler volume = 42.5%) displayed markedly lower microhardness values than did the two packable bulk fill materials (x-tra fil, filler volume = 70.1%; Tetric EvoCeram Bulk Fill, filler volume = 60–61%) and the conventional resin composite (Filtek Supreme XTE, filler volume = 63.3%). Furthermore, the behavior of the five resin composites regarding microhardness was identical irrespective of how the effect of increasing distance from the top, irradiated surface had been evaluated, i.e. microhardness obtained by measurement of profiles corresponded with those obtained using three distinct increment thicknesses. The absence of a drop in microhardness found for the bulk fill materials are in harmony with those of previous studies [17–23] and endorse the claims of the manufacturers that their bulk fill materials can be used in increments higher than the 2 mm recommended for conventional resin composites. This advantage is obtained through modifications of the monomer and filler components rendering bulk fill materials less viscous and more transparent and/or through the addition of so-called polymerization modulators or initiation boosters [18,19,24–27]. In a previous study [8] the present authors found that the bulk fill materials investigated had lower depths of cure than the 4 mm asserted by the manufacturers. The apparent conflict between those results and the present can be ascribed to the application of molds of two different materials and a difference in the age of the specimens at the time of measurement. In compliance with ISO 4049, the first study used molds of stainless steel and measured on specimens immediately after their fabrication while the present study used molds of Teflon (which absorbs much less light and allows the light emitted from the light-curing unit to pass to greater depths) and measured on specimens after a storage of 24 h with post-curing of the resin composites to various

d e n t a l m a t e r i a l s 3 0 ( 2 0 1 4 ) 1104–1112

Fig. 2 – Microhardness (Vickers hardness; HV) profiles of the five resin composites.

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Fig. 3 – Microhardness (Vickers hardness; HV) at increasing increment thickness. Different letters indicate statistically significant differences between the increment thicknesses (p < 0.05); n.s., not significantly different.

Fig. 4 – Shear bond strength (MPa) at increasing increment thickness. Different letters indicate statistically significant differences between the increment thicknesses (p < 0.05); n.s., not significantly different.

Table 2 – Distribution of failure modes after shear bond strength testing (n = 17/group). Increment thickness

Resin composite (1) Cohesive failure in dentin (%)

Filtek Supreme XTE 2 mm 4 mm 6 mm SDR 2 mm 4 mm 6 mm Filtek Bulk Fill 2 mm 4 mm 6 mm x-tra fil 2 mm 4 mm 6 mm Tetric EvoCeram Bulk Fill 2 mm 4 mm 6 mm

(2) Adhesive failure at dentin – adhesive interface (%)

(3) Adhesive failure at adhesive – resin composite interface (%)

(4) Cohesive failure in resin composite (%)

(5) Mixed failure (%)

71 35 29

0 0 6

0 0 6

0 0 6

29 65 53

53 41 59

0 0 6

0 0 0

0 12 0

47 47 35

82 65 65

0 0 0

0 0 0

0 0 6

18 35 29

41 41 47

0 0 6

0 0 0

6 18 18

53 41 29

41 47 24

0 0 0

0 0 12

12 12 29

47 41 35

d e n t a l m a t e r i a l s 3 0 ( 2 0 1 4 ) 1104–1112

extents as e.g. shown for four flowable bulk fill materials in a previous study [28]. The effectiveness of light-curing procedures has often been evaluated based on hardness measurements on the top and the bottom surface of light-cured resin composite specimens, and a bottom-to-top hardness ratio of 0.80 has been widely used as a criterium for adequate degree of cure [9,29,30]. Application of this criterium on the present results would imply that the conventional resin composite could be used in increments of up to 4 mm, and Tetric EvoCeram Bulk Fill, like the other three bulk fill materials tested, in increments of up to 6 mm. However, it needs to be kept in mind that the present results were obtained in the laboratory under ideal conditions such as a high-performance light-curing unit as well as direct access to the resin composite specimens with no distance between the tip end of the light-curing unit and the irradiated surface. Clinically, it is seldom possible to obtain such ideal working conditions and less efficient curing is to be expected. In corroboration with previous findings [12] and the second hypothesis, application of thicker increments reduced the bond strength of the conventional resin composite. However, increasing the increment thickness had no significant effect on any of the four bulk fill materials, leading to an overall rejection of the second hypothesis. This insensitivity of the bulk fill materials supports the microhardness results as well as previous results on degree of conversion and mechanical properties [18,19,22] and the depths of cure claimed by the manufacturers. When applied in an increment of 2 mm the five materials investigated resulted in bond strengths to dentin of 21–27 MPa. This range of shear bond strength values comply well with shear bond strength results previously reported for the threestep etch-and-rinse adhesive system OptiBond FL [31,32]. The percentages of cohesive failures in dentin, total or partial, were overwhelming irrespective of increment thickness or resin composite. These high percentages reflect the effectiveness of the adhesive system applied in producing such a strong hybrid layer that dentin proved the weakest link in the adhesive joint, but the percentages also reflect the imperfectness of the method [31]. Nevertheless, increasing increment thickness led to a few adhesive failures at the adhesive – resin composite interface and some cohesive failures in resin composite, indicating that light-curing at the bottom of the resin composite increments was less effective. Comparing the microhardness results (i.e. microhardness profiles and microhardness at increasing increment thickness) and the shear bond strength results of the present study, it is obvious that microhardness measurement was more sensitive in detecting an influence of increment thickness than was the shear bond strength method. Although this method has been widely applied through the last 30 years [31,33] and has the advantage of being much quicker to use than, e.g. the microtensile bond strength method, it has recently been shown to possess less discriminative power than the newer, but more complex micro-tensile bond strength method [33]. The macro-shear bond strength method might still be indicated for screening purposes and/or in the early development stages of new products, but it seems that the discriminative power of the macro-shear bond strength test is inadequate for scientific purposes, in particular when effective adhesives such as OptiBond FL are used. As hypothesized in a previous study by

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Van Ende et al., any detrimental effects on the integrity of the adhesive interface might be more apparent (and in the present study, might have led to a more distinct discrimination of bond strength) with the use of a one-step adhesive system than the use of a three-step etch-and-rinse adhesive system such as OptiBond FL [34], which can be regarded as a gold standard and has previously been described as the best performing adhesive system not only in the short term, but also in the long term [33].

5.

Conclusions

Within the limitations of the current in vitro study, the following conclusions may be drawn: • Microhardness and shear bond strength of the conventional resin composite (Filtek Supreme XTE) decreased with increasing increment thickness. • Microhardness decreased with increasing increment thickness for one of the four bulk fill resin composites (Tetric EvoCeram Bulk Fill) and remained constant for the other three (SDR, Filtek Bulk Fill, x-tra fil). • Shear bond strength remained constant with increasing increment thickness for all four bulk fill resin composites.

Conflicts of interest The authors declare no conflicts of interest, real or perceived, financial or nonfinancial.

Acknowledgments The authors would like to thank 3M ESPE, KerrHawe, and VOCO for providing the materials needed. Furthermore, we thank L. Martig, assistant at the Institute of Mathematical Statistics and Actuarial Science, University of Bern for statistical analyses and B. Rawyler, scientific illustrator at the School of Dental Medicine, University of Bern for preparation of Fig. 1.

references

[1] Sakaguchi RL, Douglas WH, Peters MC. Curing light performance and polymerization of composite restorative materials. J Dent 1992;20:183–8. [2] Pilo R, Oelgiesser D, Cardash HS. A survey of output intensity and potential for depth of cure among light-curing units in clinical use. J Dent 1999;27:235–41. [3] Filtek Supreme XTE Instructions for use. 3M ESPE; 2012. [4] SDR. Instructions for use. DENTSPLY Caulk; 2012. [5] Filtek Bulk Fill Instructions for use. 3M ESPE; 2012. [6] x-tra fil Instructions for use. VOCO; 2010. [7] Tetric EvoCeram Bulk Fill Instructions for use. Ivoclar Vivadent; 2011. [8] Flury S, Hayoz S, Peutzfeldt A, Hüsler J, Lussi A. Depth of cure of resin composites: is the ISO 4049 method suitable for bulk fill materials. Dent Mater 2012;28:521–8. [9] Bouschlicher MR, Rueggeberg FA, Wilson BM. Correlation of bottom-to-top surface microhardness and conversion ratios

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Influence of increment thickness on microhardness and dentin bond strength of bulk fill resin composites.

To investigate the influence of increment thickness on Vickers microhardness (HV) and shear bond strength (SBS) to dentin of a conventional and four b...
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