# Effects of Repolishing Systems on Surface Characteristics of a 3D-Printed Permanent Material
## 1 Introduction  
Aesthetics is one of the main objectives of restorative dentistry, which aims to restore the original natural appearance of sound teeth. If the tooth cannot be fully restored according to conservative principles, such as discoloration that is not resolvable with minimally invasive procedures or excessive loss of anatomical references, prosthetic treatment will be necessary [1, 2].  
Prosthetic restorations have undergone advancements to optimize aesthetic appeal and functional efficacy. These developments involve the integration of novel methodologies and innovative materials, enhancing the efficiency of manufacturing processes. Currently, themost commonly used materials for achieving excellent results are ceramics, like feldspathic, lithium disilicate, and zirconia [3; 4; 5; 6; 7].  
Among the other materials used in aesthetic restorations, resin-based composite resins (RBC) play a fundamental role. They already provide excellent results in direct restorations and show sufficient longevity [8], even if one of their most discussed problems is color stability.  
From a manufacturing point of view, the enhancements in CAD/CAM workflow allow the creation of indirect restorations through subtractive procedures (e.g., milling) in a clinically satisfactory and reproducible outcome associated with the use of ceramic-based materials such as feldspathic ceramics, lithium disilicate, zirconia, hybrid, and resin-based materials.  
The 3D Printing manufacturing process has recently been introduced in the dental field [9; 10]. Unlike the CAD/CAM technique, 3D printing is an additive process for creating prosthetic devices, performed by layering materials, with resin material already widely diffused and ceramic still in its beginning. This results in aesthetically improved indirect resin restorations and reduced material waste, thus increasing environmental sustainability [11] and reducing production timing and cost [12; 13]. The primary challenge associated with this technique is its recent implementation, resulting in a still limited number of studies available. At first, 3D printing in prosthetic dentistry was suitable only for temporary materials, as there were many conflicting aspects, especially about the mechanical properties, dimensional accuracy, and fit of the 3D-printed materials. More recently, permanent resin materials have been marketed [11; 14; 15; 16]  
Exposure of traditional direct and indirect resin-based materials to different staining substances, such as coffee, red wine, chlorohexidine mouthrinses [17; 18], and smoke [19], has been extensively evaluated in the scientific literature, giving a reasonably clear indication of the alterations in the resin's chromatic properties. For example, different formulations and uses of resins induce different results, and CAD/CAM resin-based materials show better chromatic stability than direct and indirect resin-based composites (RBCs) [20]. For the 3 DP resins, most of the information is still missing or scarcely addressed. Likewise, repolishing procedures may allow reverting color change below the clinical acceptability threshold in specific clinical situations, avoiding the immediate need to replace stained restorations [21]. For this aspect as well, little information is available about 3D-printed resins.  
This study evaluated coffee's ability to stain and/or alter the surface characteristics (gloss and roughness) of a 3D-printed resin for permanent restorations. Additionally, three different repolishing systems were assessed for their ability to restore color and surface properties of the 3D-printed resin material tested. The null hypothesis tested was that neither exposure of the device to a coffee-based staining solution nor different repolishing techniques significantly affected the material's color, gloss, and roughness.