## 4 Discussion  
Our null hypothesis regarding MG parameter was rejected; the open system displayed a significantly lower gap (38.4 \(\pm\) 4.4 \(\upmu\)m) compared to the closed system (55.35 \(\pm\) 4.8 \(\upmu\)m). The null hypothesis was also rejected for the AMD parameter, because the open system resulted in a significantly larger gap (196.74 \(\pm\) 9.8 \(\upmu\)m) compared to the closed system (148.14 \(\pm\) 11.3 \(\upmu\)m). It can be clearly stated that there was statistically significant difference (_p_ < 0.001) in the marginal fit between methods for both the MG and the AMD measurements. Different aspects of innovative CAD-CAM systems have been studied. The study group of [PERSON] et al., who examined the marginal gap in three-unit zirconia frameworks, found lower MG values (29 \(\upmu\)m) in the OS (milling center) compared to the values (56 \(\upmu\)m) of the CS [19]. Few studies have compared the values of marginal gap between different CS and showed better MG values with the use of CEREC inLAB: [PERSON] compared two CSs, CEREC versus CERAMILL, and found lower MG for the CEREC [22]; [PERSON] et al. compared four different CSs, CERAMILL, CERCON, LVA and CEREC and the lowest MG (37 \(\upmu\)m) was obtained by CEREC inLAB [23]. Another study by [PERSON] et al. compared three CSs: KaVo, DeguDent and Lava. The lowest MG was achieved by the Lava system (112 \(\upmu\)m) [24], although compared to the other studies, this value is higher. Comparisons between the CS and OS are described by [PERSON] et al., who showed that CSs (Cercon Brain, DeguDent) and OSs (Compartis Integrated Systems, DeguDent) had no significant differences regarding MG [25]. In 2019, [PERSON] et al. published two studies: one study which compared two techniques (CAD-CAM versus hot-press) for producing monolithic lithium dis silicate crowns showed no significant differences between the techniques in relation to the MG [12], whereas in the second, they found that zirconia crowns manufactured with CS (CEREC inLAB) displayed a significantly lower gap (85 \(\pm\) 2 \(\upmu\)m) compared to the OS (LVA milling center 133 \(\pm\) 4 \(\upmu\)m) [21]. However, data on comparisons between closed and open CAD-CAM systems are scarce. In 2018, [PERSON] et al. published an article examining the MG gaps between a Dentsply Sirona (CEREC) closed system and various open systems [26]. They reported lower mean marginal gaps (MGs) in open systems compared with closed system, thus supporting the results obtained in our study. Nevertheless, there were few limitations to the study, because they used different scanner types for each group, examined only 10 samples for each group, and measured only the vertical marginal interval (MG). However, the advantage in their study was the use of the four-axis milling unit in the closed system and the five-axis milling unit in the open system. As we mentioned earlier, research shows that the five-axis milling unit is significantly more accurate than the four-axis milling unit, and therefore the results of their study are not surprising [18; 27]. A systematic review by [PERSON] et al. [28] presented many methodologies for measuring the marginal fit of monolithic zirconia, which included the section technique [17; 29], microcomputed tomography [17; 18], and silicone technique [8]. Moreover, parameters such as MG and AMD are described differently between studies, which makes standardization difficult. Following the conclusions from [PERSON]'s and [PERSON]'s studies, we aimed to control for as many variables as possible in our study: the same researcher performed all model scans, marked the finish lines, made crown designs, and measured all the marginal gaps; we used the same scanner to perform all the scans, the same four-axis milling unit to produce all the crowns, and the same monolithic zirconia-reinforced lithium silicate ceramic block; the crown cementing protocol was the same in each sample and did not rely on a \"finger press\" as in most previous studies, but on cementing under a pressure device.