Evaluation of the Trueness of Digital Implant Impressions According to the Implant Scan Body Orientation and Scanning Method  
[PERSON]  
1 Department of Dental Education, Yonsei University College of Dentistry, Yonsei-ro 50-1, Seodaemun-gu, Seoul 03722, Korea; [EMAIL_ADDRESS]  
2 Bk21 FOUR Project, Department of Prostodontics, Yonsei University College of Dentistry, Yonsei-ro 50-1, Seodaemun-gu, Seoul 03722, Korea; [EMAIL_ADDRESS]  
2 Bk21 FOUR Project, Department of Prostodontics, Yonsei University College of Dentistry, Yonsei-ro 50-1, Seodaemun-gu, Seoul 03722, Korea; jfshim@yuhs. procedure may result in a poor fit of the prosthesis and biological and mechanical complications [7]. In addition, errors on the occlusal and proximal sides of the resulting implant crown may occur, possibly lengthening the operating time for repairing the prosthesis or causing discomfort to the patient, or even requiring remanufacturing [8]. The high accuracy of optical scanners allows digital impressions to be applied in various fields of dentistry, not only to produce inlays, onlays, crowns, and fixed partial dentures, but also to fabricate implant prostheses by capturing the three-dimensional (3D) position of the implant [9; 10]. Digital impression-taking has the advantages of creating a virtual model using an optical method, easy standardization, and high interoperator repeatability [11]. In particular, in the case of direct digitalization using an intraoral scanner, information in the oral cavity is acquired without the process of taking impressions and producing the work model. This makes the processing time efficient and also comfortable for the patient, particularly for those with a heightened gagging reflex [12; 13]. In the digital implant impression process, instead of using an impression coping, an implant scan body (ISB) is connected to the implant fixture before performing the scanning process, and information, such as the depth or direction of the implant fixture placed in the alveolar bone, is obtained. ISBs have various sizes and shapes, and the scan region has an asymmetrical shape and contains important information about the angle and orientation of the implant [14]. The position of the ISB in the dentition is recorded through optical scanning, and the library information for the ISB and the implant is used to reproduce the position of the implant fixture connected to each ISB and the abutment using CAD (Computer Aided Design) software. The reproduced data are then used in CAD software to design a prosthesis that is subsequently produced using CAM (Computer Aided Manufacturing) [15]. Factors affecting the accuracy of digital implant impressions using intraoral scanners and ISBs include the angle and depth of the implant, the implant-to-implant distance, implant location, geometry variance, scanning method, and ISB materials and designs [16; 17; 18; 19; 20; 21; 22; 23]. Most previous studies have investigated either complete or partial but multiple edentulous cases, and there have been very few studies on the accuracy of implant digital impressions according to the ISB orientation or scanning method in cases of single implants. One previous study found that the deficiency of the scanned image of the ISB affects the position of the virtual implant in the single implant case, but that study only considered a single oral scanner, and it simulated deficient scan images experimentally after completing the scanning procedure [24]. According to the International Organization for Standardization, accuracy consists of trueness and precision [25]. Trueness is the deviation of the test results from its reference value, whereas precision is the deviation between the test results. The methods of measuring the accuracy of the intraoral scanners have been either two-dimensional or three-dimensional, and the three-dimensional measuring method using superimposition is advantageous because it also evaluates local errors three-dimensionally [26].