Evaluation of degree of conversion, rate of cure, microhardness, depth of cure and contraction stress of three nano hybrid composites containing pre-polymerized spherical filler
Aim: Manufacturers aim at improving filler technology to enhance the properties of the restorative materials, thus maximising the aesthetic and functional outcome of the restored tooth. The present study tested the degree of conversion (DC), rate of cure (RC), microhardness (VHN), depth of cure (VHR) and contraction stress (CS) of three new nano hybrid composites with pre-polymerized spherical filler. Methods: Three commercially available composite resin were characterised in the present study, namely the Ceram.X\uae universal shade A3 (CXUA3), Ceram.X\uae duo enamel shade E2, and Ceram.X\uae duo dentin shade D3 (CXDE2 and CXDD3). The materials were light-cured
with a LED light (SmartLite Focus, measured output 1301 mW/cm2) following the protocol recommended by the manufacturer. DC was assessed by means of Fourier-transform infrared spectroscopy, calculating RC from a second-grade polynomial fitting of the kinetic curve. A microhardness testing machine equipped with a Vickers indenter served to measure the top and bottom VHN of 2 mm-high disc-shaped specimens, using the bottom/top surface values ratio (VHR) as indirect evaluation of the depth of cure. CS vs time was evaluated by a universal testing machine provided with an extensometer as feedback system, CS was normalized for the specimen bonding area. All data sets underwent statistical analysis with dedicated software and tested for the assumptions for the use of parametric tests. Multiple analyses of variance with
Scheff\ue9 post hoc test were carried out to compare the dependent variables of interest among the tested materials. Results: All tested materials exhibited a DC lower than
50%, with CXUA3 reaching the lowest DC value after 10 s. RC of CXUA3 at 5 s was comparable to that of CXDE2, while after 10s RC of CXUA3 decreased to a value proportional to that of CXDD3. For all the tested materials, top-VHN was greater than bottom-VHN. Top-VHN of CXDE2 was lower than CXUA3 and CXDD3. CXDD3 was the only material achieving VHR>80%. The main differences in CS among the tested materials were found during the irradiation with curing-light: CXDE2 displaying the lowest CS after 10 s and CXDD3 the highest after 30 s. Conclusion: The present study proved that the light curing protocol suggested by the manufacturer for the three composites might be improved: 10 s of irradiation seemed insufficient to adequately cure CXUA3 and CXDE2. Longer curing times for these materials appear advisable
