## 4 Discussion
It has been reported that restoratives with lower elastic modulus and coefficient of thermal expansion but higher thermal conductivity develop lower tensions on the adhesive interface, and as a consequence, restorative materials should be cautiously selected by the dental practitioner [34]. The outcomes of the current research demand the rejection of H\({}_{0}\)2 stating that after artificial aging, no differences in microgap formation would be observed among the surface-treatment groups. The present research focused on the effect of the surface modification of RBC CAD/CAM blocks on the size of the formed microgaps at the material/dentin interface. Considering that the surface pretreatment of the restorative improves the bond strength to the dentin and that microgaps are formed when there is a bonding failure, the outcomes of the current study aimed to indicate an optimal treatment for the sufficient bonding of the tested material to dentin in order to achieve long-term survival. As a consequence, the parameters that may affect the results, such as the composition of the restorative material, the resin cement, and the adhesive strategy, were eliminated, and the only variable was the surface treatment of the material. Surface pretreatments increase the impregnated surface area by the resin cement during the adhesive procedure [36]. When etching the ceramic components using hydrofluoric acid, sandblasting creates a roughened surface through the collision of Al\({}_{2}\)O\({}_{3}\) particles, while laser irradiation modifies the material's surface via the microeexplosions of water molecules in order to increase micromechanical retention. Furthermore, silane-coupling agents enhance bonding by coupling inorganic fillers with the resin matrix of the restorative material [37]. Previous studies reported that micromechanical treatments using air-abrasion can improve bonding to the surface of resin-based CAD/CAM blocks more than chemical treatments, which may partially justify the smaller size of the microgaps that were found in the current research [14; 24]. In the present investigation, mechanical surface modifications were performed using two distinguished sizes of Al\({}_{2}\)O\({}_{3}\) particles (29 \(\upmu\)m and 53 \(\upmu\)m) with the same operation parameters (air pressure, duration, and distance from the surface). These settings were chosen foraverting superficial cracks on the surface of the tested materials and to confirm a uniform surface modification [38; 39]. Hydrofluoric acid etching increases surface roughness, which is necessary for improving micromechanical retention and the wettability of the utilized coupling agent, resulting in stronger bonding [13]. Taking apart the surface layer of a glassy matrix containing silica (SiO\({}_{2}\)) and silicates (SiO\({}_{4}\))\({}^{-4}\), a porous surface is formed with a mean pore size of approximately 3-4 \(\upmu\)m that is able to be infiltrated by adhesive agents [40]. It has also been well documented that bond strength is dependent on the developing interfacial stresses between the restorative and the bonding agent, as well as the surface energy of the restorative [41; 42]. In the present study, HF induced a roughened surface similar to that reported in previous studies in polymeric silica-based CAD/CAM blocks [14]. Sandblasting treatments should be implemented with a mean particle diameter of around 50 \(\upmu\)m, because previous reports demonstrated the creation of microcracks when larger particles were used, and the time of air abrasion should not overcome 30 s [43]. More specifically, in a previous study [44], it was reported that air abrasion with 50 \(\upmu\)m Al\({}_{2}\)O\({}_{3}\) particles at 2 bar air pressure increases surface roughness up to 225% without creating surface defects, while another study [43] found that with over 30 s of air abrasion, superficial microcracks were formed, expanding up to 3 \(\upmu\)m into the material. This is consistent with the findings of the current research.