## 1 Introduction  
An accurate definitive cast, which replicates the intraoral environment, is one of the essential prerequisites for the long-term success of prosthodontic treatment [1, 2]. Different impression materials and techniques have been used to fabricate accurate complete-arch definitive casts, and there are different methods to evaluate the accuracy of these casts [3, 4, 5]. Most of the previous studies have assessed the accuracy of definitive casts by measuring the linear distances between specific landmarks [6, 7, 8, 9], however, it is difficult to evaluate the three-dimensional distortion of definitive casts due to the limitations in assessment methodology. A computerized coordinate measuring machine (CMM) was frequently used to analyze the amount of three-dimensional displacement of implant components [10, 11]. Thanks to the cylindrical shape of implant components, the centroid coordinates (x, y, z) and long axis angles of components can be computed by using a CMM. Therefore, the amount of displacement of centroids or long axis angles can be compared three-dimensionally between a patient model and definitive casts fabricated using different impression techniques. Unlike implant impressions, coordinate measurements cannot be applied to evaluate the impression accuracy of natural dentition because teeth have free-formed configurations. Several studies used a CMM to evaluate the accuracy of complete-arch impressions, however, the study placed basic geometric configurations on several tooth positions only to measure the linear distance between the configurations even though the studies used a CMM [6]. Recent development of digital technologies opened new opportunities in clinical workflow in dentistry. Digital workflows provided more patient satisfaction, as well as efficient time management in laboratory and clinical services [12; 13]. Among the digital technologies, intraoral scanners are the start of digital workflows. Currently, different intraoral scanners have been introduced and are used in clinical practice [14; 15; 16]. Previous studies have compared the accuracy of intraoral digital impression to those of conventional impression methods using elastic impression materials and modeling stone, and concluded that the accuracy of intraoral digital impression is comparable or even more accurate compared with conventional impression for small-unit fixed dental prostheses (FDPs) [17; 18]. In addition to the accuracy of small-unit FDPs, studies were also performed to assess the accuracy of intraoral digital impression for complete arch, but the results were controversial [19; 20; 21; 22]. [PERSON] and colleagues reported that an intraoral digital scan was more accurate than the conventional method for complete arch impression [23]. On the contrary, an in vivo study on complete-arch impression showed that conventional impressions using polyether were significantly more accurate than digital impression with four intraoral scanners [24]. Another laboratory study concluded that the accuracy of intraoral digital scans decreased with the increased scan length while the results of the laboratory scanner were acceptable for any scan length [25]. Most of previous studies used \"best fit alignment\" to evaluate the accuracy of digital models. Stone casts were digitally scanned using a desktop scanner to fabricate digital models, which were best-fit aligned over the digital models by intraoral digital scan. However, best-fit alignment inherently includes superimposition errors, therefore, best-fit alignment seems to be suitable for the evaluation of one quadrant, but for greater scan areas such as the complete arch [26]. Unlike partial arches, complete-arch impressions need a new evaluation method except for the best-fit alignment. The primary aim of present study was to verify the reliability of the new method to evaluate the accuracy of different impression methods, including intraoral digital scanning as well as conventional impression, without best-fit alignment but with a coordinate metrology.