## 4 Discussion  
There is no established standard method for the examination of marginal and internal fit of dental crowns. The most frequently used is the silicon replica, as it is quick, useful, non-destructive, accessible, and fit for all types of restorations. Newly digitized versions of this technique were developed, implying the scanning of the abutment and the silicon replica on the abutment. These images are superimposed and analyzed with specific software. This is a good alternative for traditional silicon replica but is more expensive because it requires an intra-oral scanner for clinical use [18,19]. The best evaluating method for crown adaptation is the invasive method, well known but with some critical issues which limit its applicability. This method implies a section in the already cemented crown embedded in resin, allowing the assessment of both internal and marginal fit, but it is suitable only for research. It was replaced by the silicon replica technique [9,20], a non-invasive and non-destructive technique, ideal for clinical applicability. It allows the clinician to perform several evaluations and measurements during the manufacturing process [1,21,22]. Recent studies investigated the differences between investigation methods. They found similar results for the cross-sectional method and silicon replica technique, with different values when compared with the triple scan method, micro-computer tomography and optical coherence tomography [23]. The researchers concluded that when evaluating the accuracy, the evaluation method must be taken into consideration for a better interpretation of the results. The prime objective of any prosthodontic treatment is to provide the patient with restorations or prostheses as precisely fitting as possible. In the case of fixed prosthodontics, the internal and marginal gaps are the essential factors to determine the long-term success of the restoration. This is why the prostheses adaptation is a subject that is still of interest for the prosthodontist and researchers [19,24,25]. A marginal opening of 120 \(\upmu\)m or less has been considered clinically acceptable, while most of the clinicians would prefer a vertical cervical gap of 60 \(\upmu\)m or less. Ideally, cemented crown margins join prepared tooth limits in perfect non-detectable junctions. Clinical perfection is not only a goal difficult to achieve, but also difficult to verify. Hence, a minimal marginal gap is nominally approximately acceptable [26]. In this study, all fabrication methods produced a clinically acceptable marginal gap with almost every value under 100 \(\upmu\)m. At the same time, SLM and milled samples reached the goal of under 60 \(\upmu\)m marginal gap, as shown in Table 1. Older studies showed higher differences between digital and conventional technologies, while recent ones suggest similar results to ours [27]. The terms marginal gap and internal gap do not have a single definition. [PERSON] et al., who established several gap definitions according to contour difference between the crown and tooth margin, states that \"the perpendicular measurement from the inner surface of casting to the axial wall of preparation is called internal gap, and the same measurement at the margin is called marginal gap\" [28]. The measurements used in this study were made according to this definition. Ultimately, the marginal and internal fit of a dental restoration provides the overarching factors for its success and longevity [5]. For instance, a superior marginal fit markedly reduces the recurrence of dental caries and the development of periodontal diseases and extends the survival of the restoration. Furthermore, an excellent internal fit is necessary to maintain and support restorations [29,30].