## 4 Discussion  
In this study, crowns fabricated with three types of ceramic blocks (HASS Rosetta, IPS e.max CAD, and VITA Suprinity) showed significant differences in the presented evaluations (trueness, surface microhardness, and marginal fit) (Tables 2-5). In addition, the trueness, surface microhardness, and marginal and internal fit showed strong positive correlations (\(p<0.001\)) (Table 6). Therefore, the two null hypotheses of this study were rejected according to the results, and the following conclusions can be drawn: the ceramic block material affects the marginal fit, surface microhardness, and trueness of the prostheses, and poor trueness and surface microhardness adversely affect the marginal and internal fit. A previous study also observed differences in trueness values according to the three types of ceramic blocks [10], and its findings agreed with those of this study: There was a significant difference in trueness according to the ceramic block material [10]. However, investigations of the correlation between trueness and hardness and their further impact on the marginal and internal fit are lacking. The findings of this study showed that poor trueness and hardness of a ceramic block material could adversely affect the marginal and internal fit of the prostheses. This study suggests that although low trueness and hardness values may result in excellent marginal and internal fit, the clinical environment should be considered. A low value of hardness for a ceramic may indicate that the material is inappropriate for use in the oral cavity [19; 21] due to the possibility of fracture as a result of high occlusal loads [19]. A high hardness value means that the ceramic material can bear high occlusal loads [19]; however, a ceramic crown with a high hardness value may cause abrasion of the antagonist [19]. Moreover, as observed in this study, high hardness values may adversely affect the marginal and internal fit. Thus, a ceramic crown must have a clinically acceptable hardness value that is not too high or too low and that provides excellent marginal and internal fit. Previous studies showed that the crowns fabricated with CAD/CAM systems had small marginal fit values of \(<\)100 \(\upmu\)m [11]. The long-term stability of a fixed prosthesis with a marginal fit value of \(>\)120 \(\upmu\)m may be affected due to periodontitis, cement dissolution, and secondary caries [11; 14]. Therefore, there is a consensus that marginal fit values less than 100-120 \(\upmu\)m are clinically permissible for fixed prostheses [11; 14]. The marginal fit values of crowns fabricated for this study were \(<\)100 \(\upmu\)m and were in the clinically permissible range for all three types of ceramic blocks. Although the three types of ceramic blocks showed differences in surface microhardness values, all were considered to be appropriate for clinical use in terms of their marginal fit. This study had several limitations. First, only three types of ceramic blocks were evaluated. New ceramic materials have been developed recently, and it will be necessary to evaluate prostheses fabricated with such products in the future. Second, this study used a four-axial milling unit, which is the type most widely used in chairside CAD/CAM. As previous findings showed that trueness evaluations might vary depending on the number of axes [7; 22; 23], it is necessary to compare these