## 4 Discussion
However, in the present study, SEM images revealed that the fillers' size (ranging between 0.1 \(\upmu\)m and 4 \(\upmu\)m) was slightly larger than what was claimed for Brilliant Crios by the manufacturer (below 0.1 \(\upmu\)m). In a two-body wear in vitro study, several CAD/CAM materials (Brilliant Crios, Lava Ultimate, Vita Enamic, Vita Suprinity), have been tested against different antagonist abraders (human tooth enamel, composite resin, feldspathic porcelain). Brilliant Crios showed maximum depth and mean maximum depth values compared to the other tested CAD/CAM materials when the antagonist abrader was enamel, while against porcelain its wear depth was slightly lower than Vita Enamic and similar to Lava Ultimate [64]. In our in vitro study, the choice of using the same restorative material as an antagonist plays a crucial role, because it mimics the clinical issues that are faced in extensive full-mouth rehabilitation involving both the upper and lower jaws, or when selecting a suitable restorative material to be placed opposing already restored antagonist teeth [46]. The Grandio disc is a nano-hybrid CAD/CAM composite, consisting of 86% inorganic fillers embedded in a polymeric matrix. According to [PERSON] [65], Grandio disc showed an adequate flexural and tensile strength. The use of different sizes of fillers enabled us to load higher amounts of strong filler particles [14]. This could also explain its positive wear resistance results. Specifically, the filler size particle reduction, as well as the high filler load, can effectively optimize the resin composite wear resistance [66; 67; 68; 69]. The SEM analysis found, for the Grandio disc, the largest particle size among the three materials tested (up to 7 \(\upmu\)m). The surface roughness may determine early wear in a two-body simulation [70]. Indeed, the polishing degree can affect the specimens' wear resistance, and a reduced wear is expected with smooth surfaces [71; 72], which may ultimately extend the longevity of a restoration. Polishing after milling or after intraoral adjustments is mandatory, to guarantee adequate clinical performance [73]. An adequate polishing should keep a surface roughness (Ra) value below 0.2 \(\upmu\)m, as it reduces bacterial adhesion [74], which is more easily achieved if the composite resin particle size decreases [75]. In this study, the roughness of each tested material surface was carefully standardized, subjecting all specimens and abraders to the well-known and clinically accepted polishing procedures under highly controlled laboratory conditions. Following the SEM analysis, Ena Cad had the smallest and the most homogeneous filler particle size range compared to the rest of the CAD/CAM dental resin composites tested, which suggests potential aesthetic advantages and better polishing. The wear resistance was the only mechanical property analyzed in this in vitro study. Additional tests, including Vickers microhardness, a flexural strength test or compressive strength test, might provide further interesting information, which is useful to better characterize the mechanical behaviors of the examined materials. The complexity of the oral cavity concerning temperature, humidity and pH changes is hardly reproduced by an in vitro test [76; 77]. The present results showed sample vertical wear mean values ranging between 0.061-0.086 mm, which is approximately twice the intraoral physiological enamel wear per year (0.035 mm) reported in previous papers [1]. However, it must be considered that in vivo wear is a complex mechanism that may strongly vary from patient to patient [78]. According to previous research [49; 50; 51; 79; 80], in the present study the Ivoclar two-body wear test method was used, and this is based on the simulation of 120,000 chewing cycles. Some authors stated that, in clinical conditions, approximately 330,000 chewing cycles are registered in one year of mastication [51]. This would mean that 120,000 cycles should correspond to 132.7 days (approximately 4 months) of clinical chewing strokes [45].