## 4 Discussion  
In recent years, digitized dental arch models have become a major trend in dentistry. More and more imaging devices enabling digital data acquisition from impressions and plaster models have become available [7]. Accordingly, the accuracy of digital models has been assessed in many previous studies [2; 10; 14; 15; 20]. Studies have generally evaluated the accuracy of the digital models obtained by scanning plaster models with various devices or compared linear measurements of digital models with the corresponding values of plaster casts [1; 2; 3; 4]. Those previous studies primarily focused on assessing the accuracy of the scanning devices, whereas we concentrated on analyzing the consequent digital products to see if they were truly accurate compared to the actual patient's dental arch. Extraoral optical scanning is currently considered an acceptable method for obtaining a digital model of the full dental arch [6; 14; 22]. Unfortunately, in this method, a plaster cast of the patient's dental arch must be made, and the errors that arise from impression-taking and plaster model pouring cannot be disregarded [7; 23]. In addition, scanning errors in the hardware and in the postprocessing algorithm increase the overall inaccuracy of the digitized data. Therefore, analyzing the accuracy of digitized models of the full dental arch compared to the patient's dental arch, rather than a plaster replica, is essential. Linear measurements were compared across the models in this study as a means of accuracy evaluation. Several previous studies have chosen to analyze measurements of surface precision [1; 6; 7; 8; 24]. In our opinion, assessments of surface precision have a high risk of distorted results. Digital model production is susceptible to minor surface flaws, such as bubbles and pearls created during impression-taking or plaster model pouring [25]. The model distortion may increase during scanning if irregularities are present on the surface. The postprocessing of STL data, known as smoothing, also overestimates errors due to surface irregularities. [PERSON] et al. [21] suggested using CMMs to obtain gold standard values of linear measurements to evaluate the accuracy of digital models. They stated that CMM measurements are highly reliable, although localizing the same landmark on digital models as on the dental cast by CMM is an error-prone process that must be overcome. [PERSON] et al. [5] also reported that the greatest proportion of errors in linear measurements occurred during the landmark-locating step. In addition, when using a small number of reference points to minimize that source of error, an insufficient number of measurements may be made to detect deviation in three dimensions, due to problems such as torsion or tilting [24]. To overcome these errors, 17 reference balls, which can be automatically detected by any measurement method, were used in the present study. The CMM probe contacted eight points on each ball and automatically found the coordinates of the centers of the reference balls. The measurement software used for the digitized models also automatically recognized the shapes of the balls and detected their centers. Overall, for both the true model and the digital models, manual errors were completely eliminated. According to [PERSON] [26], operator technique is closely related to the accuracy of impressions and plaster models. Additionally, even if a skilled dental technologist makes a plaster model according to the manual provided by the manufacturer, errors may occur due to problems during impression-taking, deformation of the material, and the model fabricating environment, such as variation in the mixing ratio, waiting time, and position of the impression body after pouring the plaster [7; 26; 27]. Since a trained technician in a well-designed laboratory fabricated the plaster model used in this study, the deviation in the size measurements of the digital model produced from the plaster model might be even greater in the normal clinical environment.