## 2 Materials and Methods  
We used 45 identical plastic teeth (FLUX 8634; Columbia Dentoform, Lancaster, PA, USA), maxillary right first molars. All were prepared by the company (with a machine) as abutments for fix partial denture with the following parameters: occlusal reduction 2.5 mm, axial wall convergence angle of 6 degrees and shoulder type finish line 1.2 mm. All 45 abutments were scanned with an intra-oral scanner (CEREC(r) AC Omnicam; Dentsply Sirona, Milford, DE, USA), and a virtual model was produced (CEREC(r) SW 4.6.1; Dentsply Sirona). The finish line was marked and the parameters for the virtual design were set as following: occlusal spacer: 160 \(\upmu\)m, proximal contacts strength: 25 \(\upmu\)m, occlusal contacts strength: 25 \(\upmu\)m, dynamic contacts strength: 25 \(\upmu\)m, radial minimal thickness: 1000 \(\upmu\)m, occlusal minimal thickness: 1500 \(\upmu\)m, margin thickness: 50 \(\upmu\)m, margin ramp width: 50 \(\upmu\)m, margin ramp angle: 60\({}^{\circ}\).  
All 45 abutments were divided into three groups (15 in each group) defined with three different radial spacer settings in the virtual design software as following: 60 microns, 90 microns and 120 microns. All other parameters were the same as mentioned earlier.  
A total of 45 crowns were ground from zirconia reinforced lithium silicate ceramic blocks (CELTRA(r) DUO, Sirona Dentsply, Milford, DE, USA) by using a grinding machine (CEREC MC XL(r); Dentsply Sirona) (Figure 1).  
The crowns were cemented to the abutments (unit) by using self-adhesive resin cement (Rely X U-200; 3M ESPE). During the cementation constant pressure was applied by using LUTRON ELECTRONIC ENTERPRISE CO. LTD FG-20 KG which was set to 50 N\(\cdot\) cm. until setting time was completed as recommended by the manufacture.  
For each unit (crown and abutment) we defined four measuring sites: Buccal, Mesial, Distal and Lingual (Figure 2). We used a new tungsten bur (FG330; Strauss&Co, Ra'anana, Israel) on each unit to define the four repeatable reference points. For using the scanning electron microscope (SEM) a further processing was done which included a gold coating (Figure 1) of the units by using mini sputter coater (SC7620; Quorum, East Sussex, UK) for 45 s.  
Figure 1: Plastic tooth of first maxillary molar with crown (zirconia reinforced lithium silicate) before and after gold coating.  
The prepared units were scanned by SEM at magnification of \(\times\)250 (JSM-IT100; JEOL, Switzerland) and the measurements were taken using operation software (InTouchScope) at the four regions of interest (B, M, L, D) in the vertical dimension at the margin of the abutment, three times at each site (Figure 3). The average of these three measurements was calculated and defined as circumferential marginal gap (CMG). All measurements were done by the same operation. For each group (15 units), 180 measurements were done and calculated to 60 results which were defined as mean marginal gap (MMG). A total of 540 measurements were taken in all three groups.  
Statistical analysis was performed using the Statistical Package for Social Sciences for Windows Release 23.0 (SPSS Inc., Chicago, IL, USA).  
Independent-samples one-way ANOVA test was used for the comparison of MMG. For multiple comparisons between the groups, a post Hoc test with Bonferroni correction was used. The statistical significance level for this work is \(p<0.05\).