to determine the mode of light transmission and its impact on the polymerisation kinetic in modern bulk-fill resin-based composites (B-RBC).
Materials and methods
Four low-viscosity methacrylate-based and one high-viscosity ormocer-based B-RBCs were considered. One material was available in three different shades that were all analyzed. Polymerization kinetic and light transmittance were assessed in 2 and 4 mm specimen depths. Incident and transmitted irradiance and radiant exposure were measured in real-time on a laboratory-grade spectrometer.
A progressive enhanced light transmittance during polymerisation was identified in all materials except for TetricEvoFlow BulkFill, which became progressively opaque in all shades. One-way ANOVA and multivariate analysis (α = 0.05) were performed. The parameter material has a significant (p < 0.001) effect on DC (η P 2 = 0.856) and light transmittance parameters (irradiance, η P 2 = 0.965; radiant exposure, η P 2 = 0.956); specimen depth influences only transmittance (η P 2 = 0.978; 0.980). DC variation in time was best described by an exponential sum function (R 2 > 0.95), differentiating between the gel- and the glass-phase and revealing a faster initiation of polymerization and a slower transition into the glass-phase by lowering the filler volume. Depth retarded the transition into the glass-phase, but did not alter DC measured 300 s post-irradiation. Moderate inverse correlation was identified among DC and filler volume% (−0.646) or filler weight% (−0.403), while no correlation among DC and light transmittance (p = 0.141; 0.125). The maximal rate of carbon-carbon double bond conversion varied within the analyzed materials but was independent from specimen’s depths.
Light transmission changes during polymerization do not alter polymerization kinetics in modern B-RBCs. DC 300s post-irradiation was maintained with depth, while light was attenuated, the faster the more translucent the material was. DC and quality of curing cannot be related to light transmittance in B-RBCs.
The bulk-fill application technique for resin-based composite (RBC) restorations gained increased interest for clinical practitioners. One reason for bulk-fill placement is a time-saving aspect, when compared to the layering process of regular RBC. Apart from well-established RBC formulations, several innovative developments, such as the incorporation of bioactive fillers or new matrix formulations (e. g. ormocer technology ), have been transferred from the regular to the bulk-fill RBCs . Yet, the clinical performance of bulk-fill restorations is insufficiently investigated. Few available clinical studies are, however, encouraging and estimate a comparable performance to restorations made by regular RBCs placed in a layering technique. Bulk-fill RBCs have been extensively analyzed in-vitro . It has been undoubtedly proven that they may be applied in increments up to 4 or 5 mm thickness , without necessitating an extended exposure time or light curing units with increased irradiance. Their mechanical properties have been shown to vary in a large range, yet comparable with regular RBCs . Based on the mechanical properties, a distinction in low- (flowable) and high-viscosity (sculptable) RBCs needs to be made, since the weak mechanical properties of the former require to finish a restoration by adding a capping layer made of regular RBCs. This extra step is undermining the time-saving benefit, but may leave more options for aesthetical adjustments. Further benefits of low-viscosity bulk-fill RBCs are the improved flowability and the higher mechanical reliability when compared to high-viscosity RBCs .
The asserted reduction in polymerization shrinkage stress in a bulk-fill application technique vs. a layering technique is not yet clearly proved . Moreover, concerns regarding a potentially higher abrasion and a forfeit in aesthetic have been raised, owing to the presence of particles with large filler sizes (>20 μm) in several bulk-fill RBCs .
Apart from implementing a new photo-initiator , the identified mechanisms to enhance the depth of cure in bulk-fill RBCs are the reduction of the filler-matrix interface by enlarging the filler size and the decrease in the amount of pigments. An optimized filler/resin refractive index mismatch in bulk-fill vs. regular RBCs to enhance light transmittance was neither specified nor demonstrated so far.
The majority of RBCs become progressively translucent during polymerization , which enables an enhanced light transmission in depth during radiation. This characteristic seems to have been modified in recently launched bulk-fill materials, which are claiming to become more opaque during polymerization, while being transparent at the beginning of the radiation. Since the quality of curing in deeper layers is crucially influenced by the amount of photons reaching these areas, an enhanced opacity during polymerization may negatively affect the polymerization kinetic and degree of cure in deeper layers. Therefore it was the aim of the study to analyse whether changes in light transmittance during polymerisation of modern bulk-fill RBCs might affect the polymerisation kinetic at clinically relevant curing depths.
The null hypotheses tested were that: a) light transmittance parameters and transmission changes during polymerization have no effect on degree of conversion (DC) and polymerisation kinetic; b) specimen depth has no effect on DC.
Materials and methods
Four flowable bulk-fill RBCs and one high viscosity ormocer-based bulk-fill RBC ( Table 1 ) were compared. Data on filler volume% and weight% were used as indicated by the manufacturer. One of the analyzed materials (TEF) is available in three different shades (IVA: Universal A shade; IVB: Universal B shade; IVW: White shade for deciduous teeth or very light permanent teeth), that were all analyzed in the present study, while all other materials offer only one shade (universal, U). Comparisons among investigated materials were performed by assessing in real time the degree of conversion (DC) and the light transmittance through clinical relevant increment thicknesses (2 mm and 4 mm).
|RBC, shade||Abbreviation||Manufacturer||LOT||Exposure time, s||Filler|
|Admira Fusion x-tra, U||AFx||Voco||1527519||20||84.0||n/a|
|Filtek Bulk Fill Flowable, U||FF||3M ESPE||N692537||10||64.5||42.5|
|Tetric EvoFlow Bulk Fill, IVA||TEF-IVA||Ivoclar Vivadent||U12113||10||68.2||46.4|
|Tetric EvoFlow Bulk Fill, IVB||TEF-IVB||Ivoclar Vivadent||T49874||10||68.2||46.4|
|Tetric EvoFlow Bulk Fill, IVW||TEF-IVW||Ivoclar Vivadent||T49987||10||68.2||46.4|
|Venus Bulk Fill, U||V||Kulzer||010108||20||65.0||38.0|
Degree of conversion (DC)
The DC of the materials investigated was measured using Fourier Transform Infrared (FTIR) spectroscopy with an FTIR-Spectrometer fitted with an attenuated total reflectance (ATR) accessory (Nicolet iS50, Thermo Fisher, Madison, USA). Profiling was performed in real-time over 5 min taking two spectra/s using two distinct specimen geometries at specimen depths of 2 and 4 mm. Teflon specimen molds (3 mm diameter, 2 and 4 mm thick) were filled in bulk with the RBCs under investigation. For the 14 groups investigated (seven materials and two specimen depths), six samples were employed to determine DC. The non-polymerized material was applied directly onto the diamond ATR crystal in the respective molds and covered with a transparent matrix strip. The materials were light irradiated (exposure times are indicated in Table 1 ) with the LED curing unit Bluephase Style (Ivoclar Vivadent, Schaan, Liechtenstein) and DC was measured at the bottom surface of the specimens. DC was calculated by assessing the variation in peak height ratio of the absorbance intensities of methacrylate carbon to carbon (C C) double bond peak at 1634 cm −1 by employing the aromatic C C double bond peak at 1608 cm −1 as an internal standard during polymerization of the uncured material using Eq. (1) . Two B-RBCs do not include an aromatic C C double bond peak (SDR and AFx) and were measured without this standard.
DC Peak % = [ 1 − ( 1634cm − 1 /1608cm − 1 ) Peak height after curing ( 1634cm − 1 /1608cm − 1 ) Peak height before curing ] × 100
For each material, the increase in DC was described by the superposition of two exponential functions where the correlation function was represented by the sum of two exponential functions as outlined in Eq. (2) .