# Effects of Repolishing Systems on Surface Characteristics of a 3D-Printed Permanent Material
## 4 Discussion  
The statistical analysis of the collected data led to the rejection of the formulated null hypothesis. The analysis revealed that exposure to the coffee solution induced significant alterations in the three color coordinates measured. It is worth noting that the three coordinates influence the perception of dental color differently. The L* coordinate (Lightness) denotes the amount of gray in a color. It is hence primarily accountable for the perception of dental color as either \"darker\" or \"lighter\", holding substantial importance in dental color perception. Conversely, the a* coordinate represents the quantity of green or red present in the color (positive values for red and negative values for green), with limited significance in the perception of dental color. The b* coordinate indicates the amount of blue and yellow within a color (positive values for yellow and negative for blue) and in dental color undergoes considerably more variation than the a* coordinate, representing the \"yellowing\" effect in dental color perception, thus playing a significant role. The staining did not significantly affect the brightness or surface roughness, and the repolishing procedure had little influence on this property, regardless of the specific method employed. Repolishing proved to be quite effective in restoring the three color parameters, even if complete restoration to the pre-discoloration values was achieved only for the b* variable and only following repolishing with the 1-step system (group 3). Remarkably, this procedure increased the initial Lightness levels. Conversely, brightness was notably decreased after repolishing with propby cup and Nupro paste (group 1). Roughness and gloss are clinically significant characteristics of materials used for dental restorations. Roughness can be described by various linear (Ra, Rq, Rz) or three-dimensional (Sa, Sq, Sz) parameters [24; 25; 26]. In this study, following previously published scientific literature [24], it was decided to measure Ra, which represents the average arithmetic value of all absolute distances from the surface profile within the measurement length. This is the most commonly used parameter to evaluate the impact of polishing techniques on restorative materials. Previous studies have demonstrated that the average Ra value of a restorative material should be below 0.2 \(\mu\)[25] to minimize bacterial adhesion [27]. In the present investigation, all recorded Ra values exceeded the threshold above, being approximately 0.5 \(\mu\) (Table 5). Nonetheless, it is noteworthy that intact human enamel typically exhibits Ra values ranging from 0.45 \(\mu\) to 0.65 \(\mu\)[27; 28]. Consequently, it can be inferred that the 3D-printed resin for prosthetic restorations tested in the study exhibited surface roughness similar to that of natural enamel and maintained this characteristic even after the discoloration test. Gloss is defined as the specular reflection from a surface and is determined according to ISO standards by comparing the intensity of incident light at a 60\({}^{\circ}\) angle with the intensity of light reflected from the surface at an equal angle but in the opposite direction. Its refractive index and surface morphology influence the material's gloss. Generally, the rougher the surface, the lower the gloss [24]. However, these two properties have different physical natures: the former being physical and the latter optical. In contrast to surface roughness, a clinical acceptability threshold for gloss has not yet been established [27]. Nevertheless, in evaluating gloss, reference can be made to the data available in the literature regarding natural enamel, considering that the composite resins judged visually similar to human enamel have been reported to exhibit gloss values ranging between 40 and 47 GU [27; 29], which is therefore considered the reference range for enamel. In this study, all gloss values measured for the tested resin (Table 6) are lower than those attributed to natural enamel.