## 2 Materials and Methods
### Trueness  
Residue was removed from each fabricated crown using an ultrasound cleaner for five minutes in distilled water. After drying, each crown was connected to a stationary jig, and the inside was scanned using a contact scanner (DS10, Renishaw, Gloucestershire, UK) (Figure 2A,B). For precision scanning, the scanner was calibrated before each use. Each scan (virtual crown) was extracted as an STL file to be used as test data for the evaluation of trueness (Figure 2C).  
The 3D inspection software (Geomagic Control X v2018.0.0, 3D Systems Inc., Rock Hill, SC, USA) recommended by ISO-12836 was used in this study. To retrieve the CDM file from the 3D inspection software and perform a 3D comparison, only the inside area of the crown was segmented, using the margin as a boundary (Figure 2D). After the CDM file was prepared, the CSM file was retrieved, and the initial alignment was conducted. The inner region of the segmented crowns was designated, and the two files were superimposed in the point cloud by best fit alignment (Figure 2E). The sampling rate was specified as 100%.  
\begin{table}
\begin{tabular}{c c c} \hline \hline
**Lithium Disilicate Block** & **Material Type** & **Lot Number/Manufacturer** \\ \hline
**HASS Rosetta** & Lithium disilicate glass ceramic & AB06 KE1502/HASS \\
**IPS e.max CAD** & Lithium disilicate glass ceramic & V36449/Ivoclar vivadent \\
**VITA Suprinity** & Zirconia reinforced lithium silicate glass ceramic & 46191/VITA \\ \hline \hline \end{tabular}
\end{table}
Table 1: Datasheet of lithium disilicate block testing.  
Figure 1: Experimental design.  
The dimensional differences between the CDM and CSM files were calculated for all data points in the segmented inner region. Each data point consists of three coordinate values (\(X\), \(Y\), \(Z\)), and the distance between each corresponding data point in the CDM and CSM files was calculated as a trueness value. The root mean square (RMS) was then calculated using the following formula [10]:  
\[RMS=\frac{1}{\sqrt{n}}\cdot\sqrt{\sum_{i=1}^{n}(X_{1,i}-X_{2,i})^{2}}.\]  
For all data points, \(X_{1,i}\) is the position of the measurement point \(i\) in the reference scan data; \(X_{2,i}\) is the position of the measurement point \(i\) in the evaluation scan data; and \(n\) refers to the number of data points measured in each analysis.  
The RMS value indicates the degree of deviation in the scan data. A low RMS value indicates a good 3D agreement of the superimposed data. Each 3D comparison is shown as a color difference map, having a range of \(\pm\)100 \(\upmu\)m (20 color segments) and an allowable tolerance range (green) of \(\pm\)10 \(\upmu\)m (Figure 2F). The red zone (10 \(\upmu\)m to 100 \(\upmu\)m) indicates that the CSM data are located above the CDM data, implying that the milling failed to reach the target position. The blue zone (\(-\)10 \(\upmu\)m to \(-\)100 \(\upmu\)m) indicates that the CSM data are located below the CDM data, implying that the milling exceeded the target position. The green zone (\(\pm\)10 \(\upmu\)m) corresponds to areas that were milled precisely. A high RMS  
Figure 2: Procedure for evaluating trueness. (**A**) The contact scanning procedure. (**B**) A 3D modeling result of contact scanning. (**C**) A crown scanned model (CSM). (**D**) A crown designed model (CDM). (**E**) A CSM and CDM superimposed. (**F**) A color difference map.  
value indicates poor 3D agreement of the superimposed data; visually, this results in larger areas of red and blue in the color difference map. Relative comparisons of the extent of the red and blue areas in the color difference maps for samples with high RMS values and the trend of these results were evaluated as in previous studies [4; 5; 7; 8; 9; 10].