## 1 Introduction  
Metal-ceramic restorations are still the most widely used for fixed prosthodontics due to their reliability and good long-term prognosis, widely tested, which has led that they are considered to be the gold standard [1; 2; 3; 4]. Base metal alloys, especially cobalt-chromium (Co-Cr) alloy, have undergone a higher development in recent decades as an alternative to the costly noble alloys and the lower biocompatibility of nickel-chromium alloys [5; 6]. In addition to good biocompatibility, Co-Cr alloys show proper corrosion stability and appropriate mechanical properties, as fracture resistance, hardness and resilience [7; 8; 9; 10]. Metal-ceramic restorations have been processed by traditional casting techniques, but currently, new prosthetic technologies have been introduced to fabricate metal-base restorations. Nowadays, Co-Cr frameworks can be processed by computer-aided design and computer-aided manufacturing (CAD-CAM), involving subtractive and additive manufacturing processes. The subtractive or milling method consists of a process controlled by a computer program, which uses power-driven machine tools with a sharp cutting tool to mechanically cut the materials and to achieve specific geometries [11; 12]. The main advantages are time-saving, the ability to create fine and precise detail, and the availability of materials [11; 13; 14; 15]. Nevertheless, it has disadvantages like the high cost and the waste of material [11; 16]. Nowadays, metal milling frameworks can be obtained by hard or soft blocks. Soft metal milling is the most recent process within the subtractive method and consists of using metal blanks in a pre-sintered state. These blanks are milled and subsequently sintered in a special furnace until achieving the proper size (volumetric shrinkage of approximately 11%) [13]. This method has the advantages of shorter milling time, fewer machine tools wear (increasing its useful life), and less risk of material contamination due to dry milling [13]. On the other hand, the additive method, especially the direct metal laser sintering (DMLS), produces the metal structures layer by layer by a high-power laser that fuses the alloy powder from a three dimensional (3D) CAD file that contains the framework's design [17; 18; 19]. The advantages include no material waste, higher productivity [11], and easy fabrication of complex shapes [18; 20]. The main drawbacks are that there may be differences in the final model production and limitations on materials so far [11; 12]. A good marginal fit is one of the main criteria to achieve long-term success in fixed prostheses restorations. The lack of an accurate fit can cause severe complications [21; 22; 23], and multiple factors, such as tooth preparation, luting procedure, prosthetic design and manufacturing technology, can affect the final adaptation of the restorations [24; 25; 26; 27; 28]. Nevertheless, despite its importance, no consensus exists in the literature on what must be considered the optimal fit value [28; 29; 30; 31; 32], and most authors continue to refer to the criteria established by [PERSON] and [PERSON] [33], which established as clinically acceptable a marginal discrepancy lower than 120 \(\upmu\)m. Currently, CAD-CAM restorations have shown high precision in their marginal adaptation, and several authors admit gaps below 100 \(\upmu\)m [28; 34; 35; 36; 37; 38]. Different methods have been proposed to measure the marginal adaptation of a restoration. Direct-view microscopic techniques are the most commonly used, although there is no consensus in the methodology and the best technique to follow [26; 27; 39]. Scanning electron microscopy (SEM) is a conservative method to provide appropriate and realistic marginal fit observations with high magnification, especially with complex margin morphologies [27; 28; 39]. Nonetheless, this method also has disadvantages as the location of reference points for measurements and the observation angle [26; 27; 28; 39; 40].