## 4 Discussion  
Based on the results of this in vitro study the null hypothesis was confirmed, namely that all of the materials included in this study were influenced by the hydrothermal aging. Based on the results, thermocycling significantly decreased the Vickers hardness. Thermocycling may cause water absorption in the resin material structure. A consequence is an expansion of the network and decreasing of the frictional forces between polymer chains. Studies made suppositions that the assimilated water would lead to hydrolysis of the silane coupling agent, altering the chemical bond between the fillers and the resin matrix [19; 61; 62]. Other studies speculated that differences in the thermal expansion coefficients of the two phases could induce stress at the network interfaces. Experiments demonstrated that an equilibrium should be reached after 30 days and further storage in water did not alter more the mechanical properties [63]. The hardness of polished samples decreased with the decrease of the inorganic filler content, both before and after thermocycling, and for glazed samples after thermocycling, because the glaze was not kept on the surface. Glaze temporary increased the microhardness, but was removed by thermocycling. This means that aging due to thermocycling affecting the materials differently in relation to surface processing and glazing should be avoided. Other studies showed that different materials behave differently under thermocycling [63]. Related to the studied materials, PICN ceramic (E) was obtained through infiltration of presintered ceramic with resin, inducing a higher density. This could be another reason that supports the higher microhardness of this material. On the other hand, resin nanoceramic materials contain hydrophobic elements such austenite dimethacrylate (UDMA), or hydrophilic elements such as triethylene glycol dimethacrylate (TEGDMA), and bisphenol A-glycidyl methacrylate (Bis-GMA), which can explain the decreased microhardness caused by an increase of the susceptibility to water sorption [13; 63; 64]. Ethoxylatech bisphenol A diglycidyl methacrylate (Bis-EMA) is a type of ethoxylatedBis-GMA that is highly hydrophilic and has no reactive hydroxyl group in its main polymer chains. It should therefore exhibit insignificant water sorption. Studies investigated composites with different filler particles and showed that those with larger fillers are more susceptible to color changes related to water sorption than those with smaller filler particles, which is due to the hydrolysis at the filler-matrix interface [19; 27; 65; 66]. All of these previous studies support the results of this study, because hydrophobic elements are included in the matrix of resin nanoceramics and the filler particles size is also small. Related to microroughness, all Ra values increased after thermocycling, but they were kept under the clinically accepted value, just below 0.1 \(\upmu\)m after the second step of thermocycling. Ra values below 0.2 \(\upmu\)m are generally clinical accepted [3; 67]. Translucency allows passing of light, and it is known as the state between opacity and transparency. Opalescence is related to the light scattering of short wave-lengths from the visible spectrum in translucent materials. This characteristic gives a material a bluish-white aspect in reflected light and an orange-brown aspect in transmitted light. Restorative materials should have similar opalescent properties to natural tooth structures. Among all of the optical properties, translucency is known as a key factor for the natural outcome of esthetic restorations [68; 69; 70; 71; 72; 73]. Previous studies registered mean TP values for 1 mm thick human enamel as 18.7 and for human dentine as 16.4 [74; 75; 76] The TP values for the studied resin matrix ceramic samples lie between 17.01 and 24.79. The OP value of the enamel-dentin complex was reported to be 4.8, and that of enamel, 7.4 [77; 78; 79]. The values for the studied resin matrix c ceramic samples lay between 4.40 and 8.05.