STOMATOLOGY EDU JOURNAL 2017, Volume 4, Issue 3 SEJ_3-2017_Online - Page 15

by integrating fibers as a reinforcing phase in resin-composites, their use as direct restorative materials underlie modern requests for fast and save application. This implies that a cavity may be restored in larger increments (> 2 mm), helping thus to reduce both the chair time and the risk to insert defects or contaminants between layers, in comparison to an incremental placement technique with conventional resin composites. 6 Currently, a FRC that is indicated to restore large posterior cavities, owing to its enhanced mechanical properties, is also conferred by the manufacturer right to use in larger increments (up to 4 or 5 mm) similarly to a bulk-fill resin composite. Whether a resin-composite is adequately cured in its deeper layers is essentially influenced by the amount of photons reaching these areas during polymerization, and consequently by the translucency of the material. It has been shown that bulk-fill resin composites may become either progressively opaquer or transparent during polymerisation, allowing hence for less or more light to reach deeper areas in relation to the light transmitted at the beginning of irradiation. 7 The change in light transmission vs. time is determined by the increasing or decreasing mismatch between the refractive indexes of monomer and filler, as the resin polymerizes. 8 It has been evidenced in commercially available bulk-fill resin composites, however, that the mode of light transmission does not alter the degree of conversion or polymerisation kinetics in depths up to 4 mm, 7 providing that the radiant exposure specified by the manufacturer was applied properly. The aim of the present study was therefore to evaluate the potential of a commercial FRC to be adequately cured in increments larger than 2 mm, and to describe the kinetic of polymerization as a function of incremental thickness. In addition, several mechanical properties, measured at micro and macro scale, were evaluated, to allow a comparison of the FRC with several particulate high viscosity bulk-fill resin composites that were measured under identical conditions and published previously. 9 The null hypotheses tested were that: a) the depth (100-µm to 6-mm) has no effect on polymerisation kinetic parameters and degree of conversion (DC) measured 300s post-irradiation 2. Materials and Methods One FRC (EverX Posterior, GC, Lot 1310242) was investigated by assessing the degree of conversion (DC) and the polymerisation kinetic at various depths (100-µm, 2-mm 4-mm and 6 mm), as well as the macro (flexural strength and flexural modulus) and micro-mechanical properties (Vickers hardness, HV, indentation modulus, Y HU and creep, Cr). In addition, the curing characteristics of the light curing unit (LCU) which was used in all tests for polymerisation (LED-LCU Bluephase 20i, Ivoclar Vivadent, Schaan, Liechtenstein, High power mode) were considered. As indicated by the manufacturer, the organic Stomatology Edu Journal matrix of the analysed FRC is methacrylate-based (Bis-GMA, TEGDMA, PMMA), while the inorganic fillers (77 % by weight) consist of E-glass fibers (5- 15 wt%) with an average length between 1-2 mm, barium borosilicate glass filler (60-70 wt%) with an average size between 0.1-2.2 µm and silicon dioxide (1-5 wt%) (http://www.dibateb.com/wp-content/ uploads/2016/01/FAQ-everx-posterior.pdf). 2.1. Degree of conversion (DC) and polymerisation kinetic. DC was measured in a real-time profile (5 minutes, with 2 spectra/s) with an FTIR-Spectrometer with an attenuated total reflectance (ATR) accessory (Nexus, Thermo Nicolet, Madison, USA). Four different specimen geometries were considered. Therefore 100 µm and 2-mm, 4-mm and 6-mm high molds (4 mm diameter) were filled in bulk. Specimens were cured by applying the curing unit for 20 s directly on the top (0 mm exposure distance) of the mold, covered by a transparent polyacetate sheet. For each thickness, six specimens were measured (n = 6). The non-polymerized composite paste was applied directly on the diamond ATR crystal in the mold as described above. DC was measured on the bottom of the specimens and was calculated by assessing the variation in peak height ratio of the absorbance intensities of methacrylate carbon carbon (C-C) double bond (peak at 1634 cm −1 ) and that of an internal standard (aromatic C-C double bond, peak at 1608 cm −1 ) during polymerization, in relation to the uncured material. Polymerisation kinetics in a fibre reinforced resin-based composite (1634cm -1 /1608cm -1 ) Peak height after curing DC Peak% = [1− ______________________________ ] x100 (1634cm -1 /1608cm -1 ) Peak height before curing The DC at the end of the observation time (300 seconds) is indicated, while the increase in DC (= decrease of the C-C double bonds) until 300 seconds post-irradiation was described by the superposition of two exponential functions as: y=y 0 +a*(1−e −bx )+c*(1−e −dx ) The term y 0 presents the y-intercept, depending e.g. on the thickness of the specimen and the composition of the material. The parameters: a, b, c, d are modulation factors of the exponential function to optimize the exponential sum function on the measured curve. In addition, the change in carbon-carbon double bond conversion with respect to time was calculated and plotted against DC. The maximal rate of C-C double bond conversion (Rate max ) and the corresponding D C are indicated. 2.2. Macro mechanical properties - Flexural strength (FS) and flexural modulus (FM) The flexural strength (FS) and flexural modulus (FM) were determined in a three-point-bending test (n = 20) in analogy to ISO/DIN 4049:1998. 10 The specimens were made by compressing the composite material between two glass plates with intermediate polyacetate sheets, separated by a steel mould having an internal dimension of 2 x 165