## 1 Introduction  
The introduction of digital technologies into dental practice has facilitated manufacturing, as computer aided design-computer aided manufacturing (CAD-CAM) enabled the fabrication of accurate definitive restorations [1]. CAD-CAM manufacturing can be subtractive (milling) [2] or additive, and the interest has recently shifted towards additive manufacturing technologies that are also known as rapid prototyping or 3-dimensional (3D) printing [3, 4, 5]. 3D-printing manufactures an object by building up consecutive layers [5, 6] and is used for a wide range of dental applications including diagnostic and master models,surgical guides, complete dentures, occlusal splints, impression trays, implants, metal crowns, copings, and frameworks [4; 7; 8; 9; 10; 11; 12; 13]. Printing has certain advantages over milling as less waste material is produced, multiple products with more complex geometries can be fabricated, and less energy is consumed [14; 15]. Moreover, due to an increased accuracy and speed, of 3D-printing has increased its popularity in the dental field [16]. Several different 3D-printing technologies, namely stereolithography (SLA), digital light processing (DLP), material jetting (MJ), material extrusion (ME), binder jetting, powder bed fusion (PBF), sheet lamination, and direct energy deposition, are currently available for polymers [7]. However, among these technologies, the popularity of DLP, which is based on the UV light activation of the photosensitive resin [17], is increasing for the fabrication of dental prosthesis [4; 18]. Interim restorations are an essential component of the prosthodonic treatment as they act as a prototype of the definitive prosthesis providing esthetics, pulp protection, tooth positional stability, and soft tissue management [19; 20]. Conventional interim restorations are fabricated commonly by using polymethylmethacrylate (PMMA) due to its accessibility, ease of fabrication and repair, low cost, biocompatibility, and stability in the oral environment [19; 20; 21]. Even though direct fabrication of acrylic resin interim crowns is feasible [22], polymerization shrinkage, possible biologic reactions due to the residual monomers, and marginal and occlusal discrepancies can be observed [20; 22; 23]. The indirect fabrication of these restorations with CAD-CAM technologies led to a better internal fit [20] and marginal integrity [24], enabling successful and long-lasting restorations [20]. Both subtractive manufacturing and 3D-printing technologies are applicable for the fabrication of interim restorations [19]. Efficiency of 3D-printing is affected by the layer thickness, laser intensity, laser speed, build angle, the geometry of the supporting structures, and printing technology [14; 21; 25; 26; 27]. Layer thickness is a controllable parameter that affects the accuracy, which is defined by trueness and precision [28], of the final restoration [7]. Therefore, setting the appropriate layer thickness is crucial to achieve optimum results. In addition, layer thickness was shown to affect the printing speed and printing accuracy [16]. In general, the layer thickness of a 3D-printer based on photopolymerization ranges between 20 to 150 \(\upmu\)m [7]. Previous studies on 3D-printed interim materials have mainly focused on the effect of printing orientation [14; 18; 25; 26; 29], while the effect of layer thickness was assessed primarily when 3D-printed dental models [16], trial dentures [30], and custom trays [13] were printed. Only one study investigated the effect of printing orientation and layer thickness on the fit of 3-unit implant-supported fixed partial dentures [31]. To the authors' knowledge, no study has evaluated the effect of layer thickness on the accuracy of 3D-printed interim crowns. Therefore, the aim of this study was to investigate the trueness and margin quality of interim crowns printed in 3 different layer thicknesses (20, 50, and 100 \(\upmu\)m) comparing with that of milled PMMA crowns.