## 4 Discussion  
Precision of indirect adhesive restorations is of utmost importance for achieving long-term prognosis, as previously discussed [31]. The present paper aimed to improve knowledge on 3D printing technology, evaluating the performance of a novel chairside 3D printer in terms of the volumetric and linear adaptation of final restorations. In the literature, various methods have been proposed to assess the precision of dental restorations, including sectioning, scanning electron microscopy, optical coherence tomography (OCT), and micro-CT. In the present paper, the authors chose to use micro-CT, as it offers a less invasive approach, and it allows both 3D and linear measurements, respectively, assessing overall condition of the interface and specific points [32]. Nonetheless, according to the recent literature, micro-CT provides high spatial resolution, enabling the detection of fine details within the tooth-restoration interface, making this method the gold standard for such evaluations [33; 34; 35]. Moreover, in the present study, a replicated tooth was used to standardize the preparation area and restoration geometry. This was done in order to avoid biases, especially in the volumetric analysis, related to interface extension, allowing a more controlled cementation phase and subsequent analysis. Moreover, it has been widely demonstrated that the accuracy of restorations is impacted by the geometric complexity and angles of the preparation design [36; 37]. The present study results showed that the CAM method significantly influenced the volumetric interfacial adaptation, with the chairside milling machine (G2) having the worst results and the chairside 3D printer the best (G1). The control group (G3), on the other hand, performed better than G2 but worse than G1. Given these data, the first null hypothesis was rejected. Moving to the linear adaptation results, the second null hypothesis was rejected since both G1 and G3 performed better than G2 in terms of internal adaptation. As a logic consequence, it is reasonable to assume that the inferior performance on the volumetric analysis is mainly related to the inferior internal adaptation of G2 rather than the marginal adaptation, which was found to be comparable in all three groups. A significant difference between G2 and G3 is expected due to the different performances of the milling units and in accordance with the literature. In fact, a study investigated the trueness of four different milling processes and found that, independently  
Figure 3: Total volume of volumetric adaptation on Mimics in sagittal and occlusal view. On the left (green volume) is shown a sample from G1 (chairside printing), in the center (blue volume) is shown a sample from G2 (chairside milling), and on the right (yellow volume) is shown a sample from G3 (industrial milling). of other steps of the digital workflow, 5-axes milling devices had higher trueness both for the internal and external restoration surfaces [38]. The tested chairside milling unit (G2) is a 4-axes machine, while the control group (G3) is a 5-axes one; thus, the results are in accordance with the previous observation [39]. Another study came to the same conclusions, explaining that the additional axis allows to process surfaces with an angle close to the insertion axis more effectively and accurately [40]. The minimum diameter of the milling burs should also be considered, which, depending on the curvatures of the CAD project, could create unwanted cavities [41]. This might be a key reason for the worse internal adaptation in G2, especially below the cuspal area, in which there is a high curvature, as shown in Figure 3. In particular, G2 was milled with 1.0 mm and 1.8 mm diameter burs compared to G3, which was made with 0.6 mm diameter burs. Accordingly, a previous study on the diameter of milling tools showed that unwanted material removal could account for the largest gap thickness in the occlusal region of milled crowns [31; 42].