The comparative evaluation of depth of cure of bulk-fill composites - An in vitro study.

INTRODUCTION: Resin-based composites (RBCs), as restorative dental materials, have given a new dimension to conservative and esthetic dentistry. The objective of the present study is to evaluate and compare the depth of cure of RBC's for posterior use: Sculptable bulk-fill composite - Tetric N-Ceram bulk fill (TNCBF), Flowable bulk-fill composites-TetricEvoflow bulk fill (TEFBF), Surefil SDR bulk fill (SDRBF), Dual cure bulk fill-Fill-Up (FDCBF) with conventional RBC-Esthet-X flow (EXF) and Filtex Z250 (FZ).
MATERIALS AND METHODS: A standardized polyacrylic mold was bulk filled with each of the six composites and light-cured for 20 s, followed by 24 h storage in water. The surface hardness was measured on the top and the bottom by recording Vickers hardness number by Vickers hardness indenter.
RESULTS: The mean bottom surface hardness value (HV) of SDR and TEFBF exceeded 80% of the top surface HV (HV-80%). Low viscosity bulk-fill composites (SDR and Tetric Evoflow) were properly cured in 4-mm increments. The TNCBF, high-viscosity composite, and Fill-Up, dual-cure bulk fill were not sufficiently cured in 4-mm increments.
CONCLUSION: With increase in incremental thickness, HV decreased for the conventional resin composite but generally remained constant for the bulk-fill resin composites. Copyright:

INTRODUCTION

Resin-based composites (RBCs), as restorative dental materials have given a new dimension to conservative and esthetic dentistry with their improved mechanical properties, clinical handling, and ability to mimic the natural appearance of teeth.[12] The placement of composite restorations is technique-sensitive and requires adequate light-curing to ensure a thorough cure.[3] If the composite is not sufficiently cured, then the function and longevity of the restoration will be compromised.[4] When restoring cavities with light-curing resin composites, the incremental placement technique (maximum - 2 mm) has been regarded as the gold standard to apply and cure the resin composite in increments of limited thickness.

While restoring cavities, especially deep ones, with 2-mm thick increments, a risk of incorporating air bubbles or failure to maintain adequate isolation leads to contaminations between the increments reduced mechanical properties of RBC restorations.[5] In addition to increased clinical time and technical complexities, other disadvantages of the incremental filling technique include reduced bond strengths as well as voids, contamination, and bond failures between adjacent RBC layers.[6]

With advances in polymer chemistry, photo-activation, and curing light technologies, a new “class” of RBCs called bulk-fill composites have emerged, that enable the restoration to be placed in 4–5 mm thick layer and cured easily and thus replacing both enamel and dentin.[78] The placement of larger increments of RBC may reduce the time needed when placing posterior restorations and thereby reduce technique sensitivity. Bulk-fill composites are available as low-viscosity flowable, for example, Surefil SDR (Dentsply Caulk) and high-viscosity restorative, for example, Tetric–N-Ceram Bulk fill (Ivoclar Vivadent, Amherst, NY) materials and dual-cure bulk fill, for example, Fill-Up (ColteneWaledent).[89]

Key properties that bulk-fill composites should possess are reduced polymerization shrinkage, a reasonable depth of cure (DOC), flowable enough to reach all the areas of the preparation without creating voids, excellent physical properties in terms of wear and function and esthetics.[210]

The DOC and degree of conversion (DC) of bulk-fill RBCs have been investigated using a variety of methods. These include the ISO scraping test, microhardness test, Fourier-transform infrared, and Raman spectroscopy. Since hardness measurement has been shown to be a practical method to indirectly determine DC for a given resin composite, hardness profiles can be used to measure DOC.[111213] The aim of this study was to evaluate the DOC of RBCs for posterior use: Sculptable bulk-fill composite, Dual cure bulk-fill composites, and Flowable bulk-fill composites with conventional RBCs.

The aim of the present work was to group different bulk-fill (sculptable, dual cure, and flowable) RBCs for posterior use in a single study [Table 1] and to compare their physicomechanical property-the DOC and microhardness under optimal curing conditions to those of two conventional composite materials chosen as references. The null hypothesis evaluated was that there would be no differences in DOC and microhardness properties between the so-called bulk-fill composites used, with two conventional composite materials chosen as controls.

Table 1

Different composite materials used in study

Test material	Composition	Manufacturer and batch number
TNCBF (high-viscosity nonflowable) Nanohybrid (n=10)	Resin: Bis-GMA, DMAFiller: Barium aluminofluoride borosilicate, glass, ytterbiumtrifluorideFiller load (wt%vol%): 80 (including 17% prepolymers)/60Mean particlle size: <1µ and nanoparticle <100 NM	IVOCLAR IVIVADENT, SCHAAN, LIECHTENSSTEIN U27917
FDCBF (Medium viscous, Microhybrid) Dual cure (n=10)	Filler: Barium aluminofluoride borosilicate glassFiller load (wt%vol%): 65/49Particle size: 0.1-5 µm	COLTENE-WALEDENTG42309
SDRBF (Bulk fill, flowable) (n=10)	Resin: Modified UDMA- (a modulator incorporated in it) EBPADMA, TEGDMACamphorquinone photoinitiator, BHT, UV stabilizer, titanium dioxide, and iron oxide pigmentsFiller: Barium aluminofluoride borosilicate glass, strontium aluminofluorosilicate glassFiller load (wt%vol%): 68/44	DENTSPLY CaulkMILFORD, DE 199630359 U.S.A07311
TEFBF (Nanohybrid bulk fill) (n=10)	Resin: Bis-GMA, DMAFiller: Barium aluminofluoride borosilicate glassFiller load (wt%vol%): 80 (including 17% prepolymers)/60	IVOCLAR IVIVADENT, SCHAAN, LIECHTENSSTEIN 139400
FZ (Microhybrid) (n=10)	Resin: UDMA and BisEMA (Bisphenol A polyethylene glycol diether dimethacrylate) have replaced majority of TEGMA.Filler: ZirconiaSilica particles.Filler load (wt%vol%): 80/60Particle size: 0.013.5 µm	3M ESPE1470A3
EXF (Liquid, Microhybrid Composite) (n=10)	Resin: BisGMA, A Urethane Modified Bis-GMA-adduct, Ethoxylate BisPhenoL A dimethacrylate, TEGDMA.Filler system: Barium fluoroaluminoborosilicate glass with an average particle size of 1 µm. Nanofiller silica with a particle size <0.02 µm.Filler load (wt%/ vol%) 60/ 38.	DENTSPLY CaulkMILFORD, DE 199630359 U.S.A50927

Bis-GMA: Bisphenol A polyethylene glycol diether dimethacrylate, BHT: Butylatedhy droxytoluene, EBPADMA: Ethoxylated bisphenol A dimethacrylate, UDMA: Urethane dimethacrylate, TEGDMA: Triethylene glycol dimethacrylate, UV: Ultraviolet, SDR: Smart dentin replacement, ADDUCT: Modified BIS-GMA adduct, TNCBF: Tetric N-Ceram bulk fill, TEFBF: TetricEvo-flow bulk fill, SDRBF: Surefil SDR bulk fill, HV: Hardness value, EXF: Esthet-X flow, FZ: Filtex Z250

MATERIALS AND METHODS

The study was performed using six RBC material-Tetric N-Ceram bulk fill (TNCBF), TetricEvoflow bulk fill (TEFBF), Surefil SDR bulk-fill (SDRBF), Fill-Up Dual cure bulk fill (FUDBF), Esthet-X flow (EXF), and Filtex Z250 (FZ) [Table 1].

Sample size calculation

By assuming that recently introduced resin composites have more the 10% increase DOC and microhardness than that of conventional RBCs, confidence interval (two-sided) 95% and power of study 80%, the sample size of our study came out to be 60. Hence, standardized samples (n = 10) of each RBC material were prepared (totally 60 samples) [Table 1].

Methodology

An opaque acrylic resin mold with a hole of 4 mm height and 6 mm diameter was prepared. The mold was placed on a glass slide covered with a Mylar strip, and then the composite was filled in bulk for each material. Other Mylar strip and glass slide were placed on top, and excess material was pressed out. The specimen was polymerized for 20s, keeping the tip of the cordless Bluephase LED light-curing unit (Ivoclar Vivadent) in contact with the glass slide (1.2-mm thick) to ensure a constant distance from the specimen. All light-curing procedures were performed with the same curing unit operating in a continuous mode while emitting a light-intensity of 1100 mW, maintained at full charge before use, and irradiance was checked periodically with a radiometer (Bluephase Meter II, Ivoclar Vivadent). After polymerization, each specimen was removed from the mold. Ten specimens of each of six composite resins used for the study were prepared (n = 10). Specimens were stored in water for 24 h at room temperature so that the unreacted monomer would leach out and not affect hardness value (HV) as is done by scrapping the uncured resin while testing with ISO 4049 method. After this, the microhardness of specimens was done. In order to prevent operator bias, this test was carried out by another operator (other than who had done the curing of composite specimens).

Microhardness test

The top and bottom surface hardness of each 4-mm high specimen were measured using the Vickers microhardness instrument (HMV-2, Shimadzu, Kyoto, Japan). The measuring diamond Indenter, the Vickers pyramid, was pressed to the composite specimen using load 0.25 Kgf for 5 s.

The DOC is the thickness of the composite that is adequately polymerized or rather as the depth where HV equals the surface value multiplied by an arbitrary ratio, usually 0.8 (HV - 80%), was calculated. Each specimen HV of the lower surface was compared with the upper surface value and was noted when it dropped below HV - 80%.[101114]

Statistical analysis

Data were entered into MS Excel and analyzed using Epi info version 7 (CDC, Atlanta, Georgia). The difference in microhardness between the top and bottom surfaces within each material were compared using paired t-tests. To compare the DOC or microhardness between different resin type's analysis of variance with Tukey post hoc testing for all six materials was used. P < 0.05 was considered for statistically significant.

RESULTS

In the current study, DOC of recently introduced resin composites was compared with that of conventional resin composites (Bulk fill Flowables-Surefil SDR and Tetric Evoflow, Sculptable/nonflowable-Tetric N-Ceram and Dual cure bulk fill-Fill-Up (FDCBF), with conventional RBCs-EXF and FZ).

The statistical analysis of HV on the top and the bottom surfaces and the bottom-to-top ratio for each test group are presented in Table 2. All of the composites showed significantly lower HV values for the bottom compared with the top surface (P < 0.05).

Table 2

Hardness value on the top and the bottom surfaces and the bottom-to-top ratio for each test group

Material	Top	Bottom	Bottom-to-top ratio
FDCBF	44.2 (2.8)A	34.56 (3.4)B	0.78
TNCBF	41.89 (3.09)A	32.56 (4.89)B	0.78
SDRBF	38.56 (3.74)A	34.38 (1.54)A	0.84
TEFBF	39.63 (2.24)A	30.17 (5.89)B	0.81
FZ	54.56 (5.67)A	41.04 (6.09)B	0.79
EXF	35.89 (4.03)A	26.36 (6.27)B	0.73

HV is detailed in mean and standard deviations. Different small cap letters indicate statistical differences between the top and 4-mm-depth bottom HV (P,0.05). A ratio of bottom-to-top surface micro-hardness over 0.80 indicates adequate DOC. Vickers hardness numbers are reported as Eg SDRBF41.89 HV.25/05, where: 41.89 is the hardness number, 0.25 indicates the load used in kgf, 05 indicates the loading time (sec) Abbrevations: TNCBF: Tetric NCeram bulk fill, TEFBF: TetricEvoflow bulk fill, SDRBF: Surefil SDR bulk fill, HV: Hardness value, EXF: EsthetX flow, FZ: Filtex Z250

A ratio of bottom-to-top surface microhardness over 0.80 indicates adequate DOC. The results show ratio of bottom-to-top surface microhardness: SDRBF (0.84) > TEFBF (0.81) > FZ (0.79) > TNCBF-FDCBF (0.78) > EXF (0.73).

Among the bulk-fill composites, the bottom surface HV of SDRBF and TEFBF, which were the bulk-fill flowable exceeded HV - 80%. However, Tetric N-Ceram and Fill-Up bulk fill did not reach HV - 80% [Figure 1].

Figure 1

Comparison of Vickers surface hardness (hardness value) of top and 4-mm bottom, as well as depth of cure (hardness value-80%), for each test group (n = 10)

DISCUSSION

Microhardness has been suggested as a way to examine the DOC of photo-activated resin composite. A value over 0.80 in bottom-to-top surface microhardness indicates adequate DOC.[15] Hardness is defined as the resistance of a material to indentation or penetration. It has been used to predict the wear resistance of a material and its ability to abrade or be abraded by opposing teeth.[1213] The HV values are highly dependent on the size, weight, and volume of the filler particles. The HV values in our study present, the average microhardness of the fillers and matrix, and for this reason, the HV value should not be considered a mechanical property and should be compared only within the same material.[10]

The DOC is the depth to which the light is able to cure the material.[311] The use of thicker increments in bulk-fill resin composites is due to both developments in photoinitiator dynamics and their increased translucency, which allows additional light penetration and a deeper cure. DOC is dependent on filler (type, size, and load), light irradiance, exposure time, radiant exposure, and also resin composition and shade. The presence of unreacted monomer within the RBC bulk may also attenuate the irradiating light, preventing the formation of free radicals and thus reducing the DOC.[10]

In the current study, DOC of recently introduced resin composites bulk-fill investigated and compared with conventional resin composites was: Flowables-Surefil SDR and Tetric Evoflow, Sculptable/nonflowable-Tetric N-Ceram, FDCBF with conventional RBCs-EXF and FZ.

The bottom-to-ratio (surface HV values) of SDR (0.84) and TetricEvoflow (0.81) bulk-fill flowables exceeded HV - 80% indicating adequate DOC. TNCBF nonflowable and Fill-Up Dual cure Bulk Fill were less than 0.80 [Figure 1 and Table 2]. The results are consistent with a recent study conducted by Jang et al. (2015) that compared DOC of various bulk-fill composites including Surefil SDR, TNCBF.

The favorable high DOC results of SDR might be attributed to the translucent matrix being highly conducible to light transmission and the incorporation of a functional photoactive group in the methacrylate matrix.[16] It has handling characteristics typical of a flowable composite but can be placed in 4 mm increments with minimal polymerization stress. SDR has a self-leveling feature that allows intimate adaptation to the prepared cavity walls. Available in one universal shade, it is designed to be overlaid with a methacrylate-based universal/posterior composite for replacing missing occlusal/facial enamel.[1718]

TEFBF contains a translucent filler and matrix that allow the light to pass through the material. It has increased working time for restoration molding as it does not polymerize quickly under ambient light. Polymerization booster are added for fast curing. The patented light-sensitivity filter technology provides expanded working time by acting as a protective shield against operatory lighting. With three universal shades featuring an enamel-like translucency of 15%, the color assortment ensures seamless blending with surrounding dentition; and the well-balanced filler composition allows the clinician to achieve a fast, easy, and high-gloss polish for an esthetically pleasing restoration.[15]

TNCBF and Tetric EvoFlow bulk-fill include Ivocerin (A dibenzoyl germanium-based photoinitiator). According to the manufacturer, it has higher photo-curing activity than camphorquinone, due to its higher absorption in the region between 400 nm and 450 nm. It can be used without the addition of an amine as coinitiator. It forms at least two radicals that are able to initiate the radical polymerization; therefore, it is more efficient than camphorquinone/amine system.[19]

FIll-Up is a dual curing, medium viscous bulk-fill composite for posterior Class I and II restorations. The dual curing mechanism is: light curing and self-curing. It is a micro-hybrid 2-component composite consisting of a base and a catalyst paste. The two components are mixed during extrusion in a static mixer and the curing starts with a defined delay after the components are brought together. The benefit of dual-cure resin materials is the ability to bulk fill the core buildup material and/or lutes an opaque restoration while minimizing the risk of light attenuation that would disrupt the setting of the deepest portions of the resin material. In the previous studies conducted the dual-cure resins which are not exposed to the appropriate amount of light or limited to only chemical curing may not obtain maximum mechanical properties due to a lower DC of the monomer.[920]

Fill-Up used in the study though did not reach the 0.80 bottom-to-top ratio. It is claimed to be able to cure up to 6 mm thickness. The chemical curing starts with a delay when the base and catalyst get mixed on extrusion in a static mixer. The material gets cured from the center and only 5–10 s light curing is required.[9]

Bulk-fill flowable exhibited large filler size with dominant polygonally shaped features compared with conventional flowable resin composites, as seen with a scanning electron microscope. The filler load was slightly increased, but the filler matrix interface was assumed to be decreased, due to the bigger size of the filler particle. Hence, it allows more curing light to transmit through the composite and improve the DOC.[21]

CONCLUSION

Within the limits of the study, flowable bulk-fill composites (SDR and Tetric Evoflow) were properly cured in 4-mm increments. The TNCBF composite and Fill-Up, dual-cure bulk fill were not sufficiently cured in 4-mm increments. For better restorative integrity in a Class II restoration the inner core of the cavity should be filled with a bulk-fill flowable first, and then, the conventional nonflowable composite on the outer capping layer should be placed. Further studies of real restorations and long-term clinical evaluations are required in this field.

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.