## 4 Discussion  
The current study used both CAD-CAM and a digital method to compare the marginal discrepancy before and after cementation. Several studies evaluating the marginal discrepancy of different luting agents have been published. There is no standardized method to measure the marginal fit, but the most common methods are the cross-sectional view, a direct view of the crown on a die, the impression replica technique and clinical examination [2]. The clinically acceptable marginal adaptation is controversial. However, [PERSON] and [PERSON] suggested that a crown would be successful if the marginal gap was less than 120 \(\upmu\)m [9]. Another result of the current study is that there was no statistically significant difference between the different measuring points among the three cement groups. However, RMGIC exhibited a greater difference in the distal marginal ridge (DMR) measurement point. This can be explained by differences in the thickness of each type of file: 10 \(\upmu\)m for GC FujiCem2 cement, 4 \(\upmu\)m for G-CEM LinkForce cement, and 3 \(\upmu\)m G-CEM LinkAce cement. It is also strongly affected by the operator's manual skills, in addition to the non-uniform preparations of the teeth in in vivo studies. During cementation, the cement space is filled. This is determined during the designing stage of CAD-CAM restorations. A study of marginal fit and internal adaptation of crowns found that a wider internal occlusal gap width favored the small marginal gap dimension [16]. A study on marginal and internal seals evaluated the adhesive interface of crowns using dye infiltration tests. They found that cement thickness did not influence the marginal seal [17]. The current study did not address this issue directly, but as shown in the results, RMGIC produced significantly more crown elevation, which may affect marginal seal. The results of the current study showed a significant difference between the resin-modified glass ionomer cement group and the two resin cement groups. However, there was no significant difference between the self-adhesive resin cement group and the adhesive resin cement group. These data are supported by other studies. In an in vitro study on lithium disilicate crowns using a profile projector, [PERSON] et al. found a higher vertical marginal discrepancy of 49 \(\upmu\)m for the resin-modified glass ionomer cement and 41 \(\upmu\)m for the resin cement [11]. Similarly, [PERSON] et al. reported a mean marginal gap of 78 \(\upmu\)m for resin-modified glass ionomer cement and 74 \(\upmu\)m for resin cement, with no statistical difference between groups [1]. Furthermore, [PERSON] et al. measured a mean increase in vertical discrepancy of 54 \(\upmu\)m after cementation for resin-modified glass ionomer cement and 36 \(\upmu\)m for adhesive resin cements, with no significant difference [8]. Another in vitro study on sectioned zirconia crowns cemented to primary molar teeth used a digital stereomicroscope and reported a marginal gap of 140 \(\upmu\)m for RMGI cement and 90 \(\upmu\)m for the self-adhesive resin cement, with no statistical difference [7]. Other studies reported different marginal gap values. An in vitro study using a scanning electron microscope found that resin cements exhibited a 144 \(\upmu\)m decrease in the marginal discrepancy and 105 \(\upmu\)m for the resin-modified glass ionomer cement, following luting in all-ceramic complete veneer crowns, with significant statistical differences [5]. However, this study observed that the mean marginal discrepancy of all-ceramic restorations was about 200 \(\upmu\)m prior to cementation, so the cement had enough space to be accommodated, facilitating better seating of the restoration. This may explain the greater marginal discrepancy values after cementation. As previously reported, there was no significant difference between the adhesive and self-adhesive resin groups [10]. However, in an in vitro study, [PERSON] et al. in, reported absolute marginal discrepancies of 186 to 195 \(\upmu\)m for the resin cement groups.