## 2 Materials and Methods  
An intact upper premolar, extracted for periodontal reasons, was selected and stored in water. After calculus cleaning and debridement with an ultrasonic device, the selected tooth was prepared by a single trained operator (more than 15 years of experience in prosthodontics) for a full-coverage indirect adhesive restoration with the following parameters: 1.5 mm occlusal reduction, 2 mm deep mesial and distal interproximal boxes with 1 mm wide butt joint margins, and 45\({}^{\circ}\) 2 mm wide chamfer margins on buccal and oral surfaces. The tooth was scanned before and after the preparation using a laboratory scanner (3 Shape E3; 3 Shape, Copenhagen, Denmark) in hi-def mode, and the obtained Standard Tessellation Language (.stl) file was exported to replicate the prepared specimen. A 3D printer was used for this purpose (Solflex 170 HD; VOCO, Cuxhaven, Germany), loaded with a light-curing resin normally used for dental casts (V-Print Model; VOCO) at the highest resolution possible. Postprocessing of the printed specimens was performed according to manufacturer instructions: after removing the support structures, excesses were cleaned off for a total of 8 min in an ultrasonic cleaning bath containing isopropanol. Then, after proper drying with compressed air, final curing was performed with a xenon photoflash unit in a nitrogen protective atmosphere (2 \(\times\) 2000 flashes; Otoflash G171, NK-Optik, Baierbrunn, Germany). The whole procedure was performed to avoid biases correlated to tooth anatomical variations. A total of 30 samples were produced and confirmed to be acceptable by two blind operators. Estimation of total sample size for the study setup with three test groups based on \(\alpha\) = 0.05 significance level was performed by means of a power analysis with the statistical power analysis program G*Power v3.1 (Open Source, HHU, Dusseldorf, Germany) in respect of an estimated effect size of 0.3 and an observed power of 0.85. After at least 48 h, an intraoral scanner (TRIOS 3; 3 Shape) was used to scan each specimen in natural light conditions. The obtained.stl files were imported into the CAD software (Trios Design Studio, version 2022.1; 3 Shape) in which an adhesive overlay restoration was designed using the unprepared tooth as a bio-copy. This allowed to obtain equal morphology and thickness in all samples as well as the same spacing for the cement that was set equal to 70 mm both in the internal and marginal area. In order to maintain as much consistency as possible among the data, a resin matrix ceramic was selected for both the procedures, according to a study: Irix(r) Max (DWS, Frankfurt, Germany) for the AM and VITA Enamic (VITA, Bad Sackingen, Germany) for the CNC milling [29]. The CAD design was exported into the three tested CAM systems, and restorations were produced according to the following materials instructions:  
* G1 (_n_ = 10): overlays were 3D-printed with a novel chairside unit (D-FAB, DWS) using Irix(r) Max (DWS) as material, then washed for 1 min in a dedicated alcohol bath and light-cured for 3 min with DCURE (DWS);
* G2 (_n_ = 10): overlays were milled with a 4-axes chairside milling machine (Cerec MCXL, Dentsply Sirona, Charlotte, NC, USA) in extra-fine mode equipped with cylinder pointed bur 12 S and step bur 12 S, using VITA Enamic as material;
* G3 (_n_ = 10): overlays were milled with a 5-axes industrial milling machine (Micro 5x, Aman Girrbach, Mader, Austria), using VITA Enamic as material, serving as control group. All machines were calibrated before the procedure, and regarding G2 and G3, milling instruments were renewed every five samples to maintain optimal cutting ability. Irix(r) Max was selected for this study since it was developed by same producer of the milling unit, thus avoiding issues and biases related to optimization of the fabrication process. Vita Enamic on the other hand, was selected due to the high amount of research related to it.