## 1 Introduction  
Breaking from the conventional dental workflow, which is dependent on the operator's experience, digital dental workflows have been made possible by the introduction of dental computer-aided design and computer-aided manufacturing (CAD/CAM) [1, 2, 3]. From a partially digital workflow that involved the process of obtaining a virtual cast through taking an oral impression, working model production, and use of a laboratory scanner [4], the use of intraoral scanners made fully digital dental workflows possible [5]. Intraoral scanners have the advantage of being able to obtain virtual casts directly from the patient's mouth without any additional work process, which is an essential element of a chairside CAD/CAM system [6, 7]. Intraoral scanners can produce 3D models and scan oral and dental soft tissues quickly and easily, and can be visualized using 3D modeling, making communication between dentists and dental technicians and patients clearer [8]. If it is difficult to use an intraoral scanner in a given dental clinic due to various oral conditions, a digital workflow is possible through partially digital workflows (the process of obtaining a virtual cast by taking an oral impression, producing a working model, and using a laboratory scanner) [9; 10; 11]. Many previous studies have evaluated the accuracy of intraoral scanners [12; 13; 14; 15]. To evaluate their accuracy, it is necessary to obtain a CAD test model (CTM), using an intraoral scanner, and to obtain a CAD reference model (CRM), using an industrial optical scanner or contact scanner with a high level accuracy so that it can be used as a standard [16; 17]. The CRM and CTM obtained using 3D inspection software undergo an optimal overlapping process, and the accuracy is analyzed through 3D comparison [18; 19]. In the analysis of the accuracy, the absolute average of the distance of all points in a cloud in the 3D modeling of the CRM and CTM is calculated, using the calculation of the root mean square (RMS) [20]. RMS values have been used for the analysis of the accuracy of scan data in many studies [21; 22; 23]. Previous studies have noted that if the scanning accuracy of virtual casts exceeds 100 \(\upmu\)m, the final restoration in the maxilla and mandible may fit incorrectly, and an allowable range for a scanning accuracy of less than 100 \(\upmu\)m has been suggested based on the acceptable cement space for the fixed prostheses [24; 25; 26]. Many previous studies have reported that the greater the scanning range, the greater the error in the intraoral scanner, and that the scanning area for long-span prostheses as the complete-arch is inappropriate for fixed prostheses [27; 28; 29; 30; 31; 32; 33; 34; 35; 36]. The limited space in the oral cavity, the effect of the accuracy of the intraoral scanner on the marginal and internal fit, and the effect of increasing the scan range on the accuracy of intraoral scanners are still to be investigated [30; 31]. In previous studies, the accuracy of intraoral scanners was evaluated in the complete-arch [36]. However, the scanning area for the fixed prosthesis of a single crown, which is most often applied in clinical practice, is between three and five teeth, and the accuracy of the abutment is considered important [27; 31]. Despite this importance, an assessment of the types of abutments used for fixed prostheses (premolar and molar) and the impacts of the volume and area of these abutments on scanning accuracy is still lacking. To aid the use of intraoral scanners in various dental clinical treatment environments, such studies need to be conducted in dental clinics. Thus, the present study aims to analyze the accuracy of two types of abutments (premolar and molar) for fixed prostheses, using three types of intraoral scanners (CS3600, i500, and EZIS PO) and one type of laboratory scanner (3 shape E1). The study also analyzes the impacts of the volume and area of the abutments on scanning accuracy.