## 2 Materials and Methods  
The present study involved the design of crowns using CAD software (crown-designed model (CDM)), the production of crowns using milling machines, the digitization of the inner surface of the crowns using a touch-probe scanner to obtain a crown scanned model (CSM), and overlapping of the CDM and CSM using testing software for analysis. A 3D analysis was also conducted (Figure 1). To determine the sample size, a pilot experiment was conducted five times. A power analysis (G*Power v3.1.9.4, Heinrich-Heine-Universitat, Dusseldorf, Germany) revealed that an appropriate sample size would be 20 (actual power = 98.93%; power = 98.9%; \(\alpha\) = 0.05). A single crown was designed by dental CAD software (EZIS VR, Digital Dentistry Solution, Seoul, Korea). The default values recommended by the manufacturer were set for the inner and outer surfaces of the crown. In addition, the designed virtual crown was extracted as a stereolithography (STL) file (CDM) to enable its use as the base data for the trueness evaluation. To produce the STL file for the designed crown, it was moved to a milling unit (EZIS HM; Digital Dentistry Solution, Seoul, Korea). The EZIS HM is a chairside four-axis milling unit. A crown milling path based on the STL file was set in the CAM software (EZIS VR; Digital Dentistry Solution, Seoul, Korea). According to the manufacturer, it offers a processing accuracy of \(\pm\)10 \(\upmu\)m and a maximum spindle speed of 63,000 rpm and can produce a crown at a rate of 15 min/unit. In addition, for the production of a more precise prosthesis, the manufacturer calibrated the milling unit. In addition, three types of lithium disilicate blocks (e.max CAD, HASS Rosetta, and VITA Suprinity) were used: The data for these three types of lithium disilicate blocks are listed in Table 1. The lithium disilicate block was mounted in the milling unit and processed (Figure 2A). A pilot test revealed that there was no difference in the trueness of five crowns processed using one burr instrument (Step Bur 12 and Cylinder Pointed Bur 12s, Dentsply Sirona, York, PA, USA) (\(P\) = 0.172). For the processing, the burr was replaced five times. Figure 1: Experimental design. Any residue was removed from the produced crown by washing it in distilled water for 5 min using an ultrasonic cleaner. After drying, it was mounted in a jig, and its inner surface was scanned using a touch-probe scanner (DS10, Renishaw plc, Gloucestershire, UK) without a crystallization firing cycle (Figure 2B). The touch-probe scanner uses a probe, which gently touches the surface and is capable of rising and falling through 200 \(\upmu\)m. The inner surface of the crown milled using the 1.0 mm milling burr was precisely scanned using a probe with a diameter of 0.5 mm. The touch probe was used to scan from the inner end of the crown to the end of the crown margin. The coordinates of about 20,000 points on the inner surface of the crown were recorded by the touch probe. Since the use of this method did not incur any errors caused by the optical characteristics of the object, it was ideal for scanning the inner surface of a lithium disilicate crown. In addition, the trueness could be analyzed precisely because of the excellent repeatability/reproducibility that was possible with this instrument [39]. To ensure accuracy, the touch-probe scanner was calibrated prior to each scan. All of the scanning processes conducted as part of this study were conducted at an ambient temperature of \(23\pm 2\)\({}^{\circ}\)C in accordance with international organization of standardization (ISO)-12836. Since errors may occur depending on the operator [27; 40], a single skilled operator (K.S.) performed all of the scans and analyses. In addition, the scanned virtual crown was extracted as an STL file to enable its use as test data in the trueness evaluation (CSM). The 3D analysis program used in this study was Geomagic Company's 3D inspection software (Geomagic control X v2018.0.0, 3D Systems Inc, Rock Hill, SC, USA), as recommended by ISO-12836 [27].