Evaluation of the degree of conversion and rate of cure of a new BIS-GMA-free nanohybrid composite

Aim: The polymerization efficiency of the newintroduced Bis-GMA-free nanohybrid composite containing pre-polymerized spherical filler Ceram. X®Duo-D3 (CXDD3, Dentsply) was investigated in terms of degree of conversion (DC) and rate of cure (RC). The obtained results were compared with those of two commonly used composites of equivalent dentin shade: the nanohybrid Tetric EvoCeram®-A3.5 (TECA3.5, Ivoclar Vivadent) and the nanofilled FiltekTMSupreme XTE-D3 (FSAD3, 3M ESPE). Methods: DC was evaluated using a Fourier Transform Infra-Red Attenuated Total Reflectance equipment (FTIR-ATR, Nicolet 6700-ThermoFisher) recording the photopolymerization kinetic (1 spectrum/s). Each composite (N=10) was placed on the FTIRATR diamond stage in a 2-mm thick silicon mold and photopolymerized for 40s with the LED curing-light SmartLite Focus®(Dentsply). Infrared (IR) spectra were obtained between 4000cm−1 and 500cm−1 at a resolution of 16cm−1. DC was calculated as: DC = [1 - (R40)/(R0)] * 100. Where R0 represents the ratio between the intensity of the reactive group signal (C=C, 1635cm-1) and the internal reference (C=O, 715cm-1) of the unpolymerized material and R40 the same ratio after 40s of photopolymerization. The kinetic trend was obtained plotting DC vs. time for all the materials. The kinetic curves were fitted with a second-grade polynomial expression, from the first derivative of the fitting, the RC at selected times (5, 10s) was calculated. RC≤0.5 between two consecutive times was considered as arbitrary value for the curve plateau.All data were subjected to statistical analysis with dedicated software (Statistical Package for Social Sciences Software v.15.0, SPSS Inc.) and tested for the assumptions for the use of parametrical tests: the normality of the distribution and the equality of variances were verified with a Shapiro-Wilk test and a Levene test, respectively. The influence of the materials’ composition on the DC and RC values was evaluated by one-way multivariate analysis of variance (ANOVA), pairwise comparison between groups were performed with a Scheffé post hoc test at a preset α=0.05. Separately, RC values registered for each material after 5 and 10s of curing time were compared with a paired-samples t-test. Results: For all tested materials, the obtained DC after 40s of photopolymerization was lower than 50%. Among the tested composites, CXDD3 showed the highest DC (p < 0.05) after 40s of photopolymerization, while TECA3.5 and FSAD3 reached lower DC values, not significantly different between each other (p>0.05). Within each composite, RC at 10s resulted significantly lower than that at 5s (p<0.05). Among the three composites, RC values, both at 5 and 10s of photopolymerization, were significantly different (p < 0.05) following the trend: CXDD3>TECA3.5>FSAD3. CXDD3 showed the faster kinetic reaching the plateau (RC≤0.5) after 25s. Differently, FSA3D and TECA3.5 showed a significantly slower kinetic (p<0.05): TECA3.5 kinetic trend reached the plateau after 30s of photopolymerization while the RC of FSA3D slightly decreased with time with a pseudo-linear trend never reaching the RC value set as plateau. Conclusions: Within the limitation of the present in vitro study, it is possible to assess that DC and RC of the studied composites were materialdependent: CXDD3, characterized by a Bis-GMA-free composition, achieved higher DC thanks to a faster kinetic. However, considering that for all the tested composites the average DC remained below 50% after 40s of photopolymerization, an increase of the curing-time or a reduction of the layer thickness may be suggested in the clinical setting of the dentine shaded composites.