## 4 Discussion  
Our null hypothesis was rejected, as we found that the marginal gap for a radial spacer of 60 microns (162.99 \(\pm\) 16.25 \(\upmu\)m) was significantly higher compared to a radial spacer of 90 (41.85 \(\pm\) 3.57 \(\upmu\)m) and 120 (41.85 \(\pm\) 5.3 \(\upmu\)m) microns. Marginal fit can be measured by different methods; replica technique, micro-CT, sectional technique and direct visualization are some of these methods [8; 12; 13; 14; 15]. In our study we used the direct visualization method. In the past, when using a die model for metal crowns, the spacer was adjusted to 30-40 microns for ideal cementation using ZPC; the frictional fit between the abutment and the crown depends on the spacer and can affect the retention [16; 17]. Nowadays, when \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline  & & & & & & \\ \hline
**(I) Spacer** & **(J) Spacer** & **(I-J)** & **Std. Error** & **Sig.** & **Lower Limit** & **Upper Limit** \\ \hline
60 microns & 90 microns & 121.14 & 14.26 & 0.00 & 85.58 & 156.69 \\  & 120 microns & 121.13 & 14.26 & 0.00 & 85.58 & 156.69 \\ \hline
90 microns & 60 microns & \(-121.14\) & 14.26 & 0.00 & \(-156.69\) & \(-85.58\) \\  & 120 microns & 0.00 & 14.26 & 1.00 & \(-35.56\) & 35.56 \\ \hline
120 microns & 60 microns & \(-121.13\) & 14.26 & 0.00 & \(-156.69\) & \(-85.58\) \\  & 90 microns & 0.00 & 14.26 & 1.00 & \(-35.56\) & 35.56 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Multiple comparisons with dependent variable of MMG (\(\alpha\) = 0.05). Figure 4: Box plot of all three groups. \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline  & & & & **95\% Confidence Interval** & & \\ \hline
**Spacer** & **Mean** & **SE** & **Lower Limit** & **Upper Limit** & **Min** & **Max** \\ \hline
60 (\(\upmu\)m) & 162.99 (\(\upmu\)m) & 16.25 (\(\upmu\)m) & 128.13 (\(\upmu\)m) & 197.85 (\(\upmu\)m) & 59.22 & 300.18 \\ \hline
90 (\(\upmu\)m) & 41.85 (\(\upmu\)m) & 3.57 (\(\upmu\)m) & 3  
using a digital work flow the radial spacer can be set according to the material and to the manufacturer recommendations. In our study we used CELTRA(r) DUO and we could not find the manufacture recommendation for the ideal radial spacer. We assumed that as the radial spacer increases the marginal gap decreases. [PERSON] et al. examined the influence of radial spacer (25, 30, 40, 50 microns) on the marginal fit of monolithic zirconia crowns and showed that as the radial spacer increases the marginal fit decreases; the lowest marginal gap was at the group of 50 microns. In this study, a stereomicroscope was used to measure the gap. We used a SEM which has a larger magnification and better accuracy. Moreover, in our study we checked higher radial spacers (60, 90, 120) and were able to show that a radial spacer of 90 microns showed the same marginal gap as 120 microns and therefore should be set as the optimal spacer [18]. A study by [PERSON] et al. used lithium disilicate crowns with three different radial spacers (50, 100, 200 microns) and also showed that as the radial spacer increases the marginal gap decreases. The limitations of this research include different teeth preparations, the space gap was filled with PVS and the pressure was set by finger and not standardized using a controlled pressure machine, all of which may affect the results [19]. Similar results were obtained when using PMMA crowns with three different radial spacers (40, 50, 60 microns) [20]. It should be noted that in our study a 60 microns radial spacer gave an unacceptable clinical marginal gap (162.99 microns) compared to an acceptable marginal gap at 50 microns that was shown in the studies by [PERSON] and [PERSON]; we assume that this difference is mainly due to the cementation in our study and also due to different measurement method (stereomicroscope versus SEM) and different materials (monolithic zirconia and glass ceramic versus zirconia reinforced lithium silicate).