## 1 Introduction  
Indirect restorations precision, both in terms of marginal and internal adaptation, allows a low cement volume, improving the mechanical stability and durability of the tooth-restoration interface [1, 2]. It is well known that the lack of adequate marginal fit can lead to tooth sensitivity, plaque accumulation, cement dissolution, microleakage, and recurrent caries [3]. One study suggested that marginal gap values of 50 \(\upmu\)m to 100 \(\upmu\)m are acceptable [4]. Although a clinically acceptable marginal gap value remains unclear [5],the literature agrees in defining clinically acceptable gap values around 120 \(\upmu\)m with modern resin-based cements [6]. However, it has also been demonstrated that internal mis-adaptation can negatively affect the fracture resistance of the tooth-restoration complex, especially if combined with the progressive degradation of the luting cement with function [7; 8; 9]. Therefore, proper marginal and internal adaptation is crucial for a good long-term outcome independently from other variables [10; 11].  
Some years ago, precision was completely delegated to dental technicians, with the premises of a proper dental preparation and impression. Nowadays, in the era of digital dentistry, clinicians have the possibility of producing their own restorations in their practices through chariside procedures. Even if it is true that computer-aided design/computer-aided manufacturing (CAD/CAM) systems have made single-tooth indirect restoration easier, faster, and predictable, concerns about the final quality outcome of the procedure can be raised [12; 13].  
Among chariside manufacturing processes, subtractive computer numerically controlled (CNC) milling is the most widespread and commonly used technique to produce indirect restorations [14; 15; 16]. Nevertheless, CNC milling has some intrinsic limitations. Machining of fully sintered blocks confers dimensional accuracy to the restoration, but it might introduce microscopic cracks on the ceramic surface, affecting the durability of the restoration [17]. Secondly, the reproduction of intaglio surface geometry and occlusal anatomy is dictated by several parameters, among which are the size of the milling burs and the number of working axes of the milling machine, meaning that the precision will be strictly machine-dependent [18]. Lastly, milling or grinding allow fast and effective fabrication but are also associated with high material loss and instrument wear [19; 20]. For these reasons, great interest has been recently raised towards additive manufacturing (AM), which has recently been integrated into dental practices, with more and more options becoming available [18; 21; 22; 23]. All AM methods have several benefits in common, such as decreased manufacturing time and reduced waste of raw materials [24]. In addition, the restoration's layer-by-layer composition avoids milling-related stresses and allows chromatic variation [25]. Moreover, AM allows to reproduce complex geometries and fine details of surface preparation independently of axes or burs size and boasts the advantages of needing less equipment and having lower production costs [18; 26]. On the other hand, limitations of AM technology include the necessity of polymerization at the end of the printing phase, the wide amount of different available technologies, and the presence of supports that must be properly positioned [23].  
Given these premises and to the best of authors knowledge, few data are available regarding chairside AM precision, especially when compared to conventional CNC milling-processed restorations [27; 28; 29]. Thus, the aim of the present study was to evaluate the volumetric and linear interfacial adaptation of an indirect adhesive restoration fabricated with chairside CNC milling and a novel chairside 3D printer through micro-computed tomography (micro-CT). The null hypotheses tested were that (1) volumetric interfacial adaptation and (2) linear adaptation are not influenced by the manufacturing technology employed.