## 4 Discussion  
According to the findings of recent studies, the development of chairside CAD-CAM has enabled the creation of accurately fixed prostheses in a short time. However, the results of the present study revealed that the trueness of the produced crowns might vary depending on the type of lithium disilicate block used. In a clinic, a crown should be designed while considering the difference in trueness that arises as a result of the type of lithium disilicate block used. For example, when using CAD software to design a crown, a higher space value should be given to e.max than to HASS and VITA, or the inner surface should be further adjusted after production. If this compensation is not applied, it may take much time for the final fitting and adjustment of the prosthesis in the oral cavity. Based on the results of the present study, we can conclude that the trueness may differ depending on the position of the inner surface of the crown. In particular, the red zones tended to dominate (10-100 \(\upmu\)m) in the angular region. This implies that the milling failed to reach the target position. The reason for this has been identified in previous studies. [PERSON] [28] noted that the number of mill burrs affects the accuracy, with the trueness being better when three burrs were used, relative to when two burrs were used. The use of three burrs produced a better trueness than when only two burrs  
Figure 4: Comparison of color difference maps for different lithium disilicate blocks. Figure 5: Comparison of color difference maps for five views. were used because the use of three burrs led to a greater range of burr diameters. A smaller-diameter burr could be used to more precisely mill angular regions. [PERSON] [13] found that the trueness was lower with a five-axis milling machine than with a four-axis machine. This is because the stiffness of a milling machine decreases as the number of axes increases. In regions with, for example, line angles, processing using a four-axis milling machine is difficult due to the entry angle and the size of the burr. In the present study, a four-axis milling machine of the type that is often used chairside was used. We judged that it would be possible to compare it to a five-axis milling unit if we were to refer to the results of previous studies. Since different types of lithium disilicate blocks vary in terms of their cutting rates, we could assume that the trueness would differ depending on the type of lithium disilicate block used. Since our study controlled for various variables (replacement of the milling burr, the use of a milling machine with high accuracy (\(\pm\)10 \(\upmu\)m), the use of a touch-probe scanning process in which the ceramic material did not scatter the light, and a sufficient sample size with an actual power of 98.93%), this assumption was reliable. If the hardness of the material themselves is low, the cutting rate is greater, and hardness varies depending on the product. The milling conditions (precision, spindle speed, number of axes) depend on the type of milling unit. Further studies should be conducted to analyze the differences between the biomaterials tested. According to [PERSON] [41], the feed rate and depth of cut may differ depending on the type of block, with lithium disilicate ceramic being the most difficult of the ceramic materials to process. However, for this hypothesis to be reliable, we should undertake a subsequent study that measures the surface hardness of the lithium disilicate block and analyzes its correlation with the trueness. The inner trueness of a lithium disilicate crown was evaluated without a crystallization firing cycle. The crystallization firing cycle causes some shrinkage [42] and can affect the trueness. The method described herein did not involve a crystallization firing cycle. This was done to exclude any variables that could occur in the firing cycle. However, further studies are needed to evaluate the trueness after the crystallization firing cycle to better approximate actual clinical practice.