## 4 Discussion
The use of a resin directly layered onto the surface of the restoration (of the specimens in the present case) followed by a polymerization carried out in a curing unit with a nitrogen environment showed a mean Ra of 0.25 \(\upmu\)m--thus a very smooth surface, and smoother than the reference enamel range, and statistically significantly better than all the other systems. Some references are available in the literature for the evaluation of roughness. Concerning plaque retention, the reference value reported in the literature for Ra is 0.20 \(\upmu\)m [22; 42]. In this regard, only the group in which a resin layer was polymerized in a curing unit with nitrogen was able to obtain a Ra value close to this threshold. Tongue sensitivity, which is the capability of the tongue to perceive the roughness of a restoration, has been reported as 50 \(\upmu\)m. For this threshold, several systems tested were below or close to this threshold. Only OS, SW, and HL showed a Ra well above the referenced value. The Ra value over which a restoration is considered to generate wear on the antagonist is reported to be 1.50 \(\upmu\)m [43]. All the tested systems were able to finish/polish the surfaces well below this limit. The results of the various materials compared with the reference values are reported in Figure 4.  
Similarly to roughness, the F&P procedures are also aimed at providing an enamel-like gloss. This value has been reported to be 53 GU by [PERSON] et al. [44], while [PERSON] et al. [45] indicated the final gloss of a resin composite to be in the range of 40-53 GU. All these values should be cautiously evaluated, as no agreement has been achieved in the literature on the measuring setup for gloss, particularly concerning the geometry of viewing. This fact determines a lack of uniformity among different studies and makes a direct comparison of the findings not completely reliable. Some authors reported that a 20\({}^{\circ}\) viewing geometry angle enables an improved differentiation compared with a 60\({}^{\circ}\) angle [46], while other authors reported the 45\({}^{\circ}\) angle as the ideal geometry [46]. [PERSON] and [PERSON] [47], using a 60\({}^{\circ}\) measurement angle considered closer to the surface observation angle by an average person, reported a classification where they defined as \"poor\" a finishing below 60 GU, as \"acceptable\" a finishing in the range of 60-70 GU, as \"good\" a finishing in the range of 70-80 GU and as \"excellent\" a finishing above 80 GU. In the present study, in agreement with [PERSON] and [PERSON] and also following the technical report of ISO 2813-2014 [48], a 60\({}^{\circ}\) geometry was used. Therefore, according to the reported classification, all the F&P systems tested achieved a \"poor\" finishing in terms of gloss, with the only exception of the NG group, which obtained a mean of 90.7 GU. This value, together with showing a statistically significant difference with all the other systems tested, makes NG the only group that could be classified as \"excellent\". It should be pointed out that as the paper by [PERSON] and [PERSON] is not a clinical dental paper, an \"exceeding\" GU value had not been considered. This should probably be taken into consideration as if an opaque surface differs from human enamel, a mirror-like surface would probably also show a non-natural outcome (Figure 5). Figure 4: Mean Ra of the tested groups compared with non-treated specimens mean and literature reference values for enamel, plaque retention, tongue sensitivity, and antagonist wear. Figure 5: The mean Ra of the tested groups compared with non-treated specimens mean and literature reference values for the evaluation of the effectiveness of gloss according to [PERSON] and [PERSON] [47]. The present study evaluated eight different F&P systems quite different from each other in terms of steps, procedures, and equipment. Therefore, the ease of use and time-related aspects should also be mentioned.