Investigations towards nano-hybrid resin-based composites.

OBJECTIVE: Clinical data indicate an increased trend in material fracture as reason for failure in composite restorations, questioning whether modern resin-based composites (RBCs) are able to fulfil the rising aesthetical demands and to provide at the same time a sufficient mechanical stability also in larger cavities. Nano-hybrid RBCs are promoted as materials with improved mechanical properties. The aim of this study was to analyse differences in mechanical properties within and between modern flowable and non-flowable nano-hybrid and micro-hybrid RBCs by measuring mechanical properties at macro- and micro-scale. METHODS: Thirty-four RBCs with traditional and new monomer formulation or photo-polymerization initiator technology-15 nano-hybrid, nine micro-hybrid and ten flowable-were therefore considered. Flexural strength, flexural modulus (E(flexural)), indentation modulus, Vickers hardness (HV) and creep were measured after the samples had been stored in water for 24 h at 37°C. Differences within the materials as well as within material categories were statistically analysed using one-way ANOVA with Tukey HSD post hoc test (α = 0.05) as well as partial eta-square statistics. RESULTS: The category of micro- and nano-hybrid RBCs performed in all properties superior compared to the flowable RBCs. The former two categories differ significantly only with regard to three parameters, with nano-hybrid RBCs showing higher HV respectively lower E(flexural) and filler weight. The micro-mechanical parameters proved to be more sensitive to differences in filler amount and RBCs type than the macro-mechanical properties. CLINICAL RELEVANCE: Only few differences were found between nano-hybrid and micro-hybrid RBCs as a material category and thus, from laboratory tests, no clear advantages in the mechanical stability in stress-bearing areas of nano-hybrid RBCs are expected clinically. Similar is valid for materials with new monomer formulation or photo-polymerization initiator technology. However, several of the measured nano-hybrid RBCs showed consistently higher mechanical properties than the mean values of the micro-hybrid RBCs.

Curing efficiency of modern LED units.

Recent reports claim that modern light-emitting diode (LED) curing units improve curing efficiency by increasing the units' irradiance. In this context also, short polymerisation times up to 5 s are proposed. The aim of this study was to examine whether there are differences in the curing efficiency of modern LED curing units by assessing their effect on two different composite materials and by varying the irradiation time. A nano- and a micro-hybrid resin-based composite (RBC) were polymerised for 5, 10 and 20 s with three commercial and a Prototype LED unit (Elipar™ S10). Cylindrical specimens (6 mm in depth, 4 mm in diameter) were prepared in three increments, each 2-mm thick, and were consecutively cured. Degree of cure was measured for 20 min in real time at the bottom of the samples, starting with the photoinitiation. The micro-mechanical properties (modulus of elasticity, E and Vickers hardness, HV) were measured as a function of depth, in 100-µm steps, on the above described samples stored in distilled water for 24 h at 37°C. Data were analysed with multivariate ANOVA followed by Tukey's test, t test and partial eta-squared statistics. In descending order of the strength of their effect, the type of RBC, depth, polymerisation time and curing unit were significant factors affecting the micro-mechanical parameters (p < 0.05). The degree of cure at 6-mm depth was less but significantly influenced by the curing unit and curing time and was independent from the type of RBC. A 5-s irradiation time is not recommended for these units. Whereas a 5-s irradiation is acceptable at the sample's surface, a minimum of 20 s of irradiation is necessary for an adequate polymerisation 2 mm beyond the surface.