A typodont tooth (tooth 36) was prepared to receive an all-ceramic crown restoration. The preparation had a circumferential shoulder margin, an axial reduction of 1.5 mm, a 1.5 mm occlusal reduction, rounded internal angles, and 12° total occlusal convergence angle. The crown was scanned (CEREC Omnicam, Dentsply/Sirona, Germany) and printed in Cobalt-Chromium. For the crowns, the milling parameters were set to produce an internal gap of 200 µm. The materials used were Sirona blocs C (Dentsply Sirona, Germany) and Zirconium oxide (inCoris TZI puck, Dentsply Sirona, Germany). The MC X5 milling machine (Dentsply/Sirona, Germany) was used to mill all crowns.
Diamond burs (set of 1.2, 1.4 and 2.2 mm sizes, Dentsply/Sirona, Germany) were used to mill the Sirona blocs C. Tungsten Carbide burs (set of 0.5, 1.0 and 2.5 mm sizes, Dentsply Sirona, Germany) were used to mill the inCoris TZI puck.
Prior to milling, and after the 10th, 20th and 30th milled crowns the milling process was paused, and the burs were scanned by Scanning Electron Microscope (SEM) (Carl Zeiss Sigma Filled Admission Zeiss, Germany), to count the particles lost after a method modified from Yara et al (2005). They were then re-installed to continue milling the crowns. The Zirconia burs were similarly scanned in order to determine any edge wear.
For the diamond burs, the bur shank was marked into four quadrants around its circumference to ensure that measurements were made in the same quadrant each time. The burs were scanned with magnifications of 100x, 200x and 500x. The SEM images of the burs were imported into a computer graphics programme (CorelDraw, Corel, Canada). A frame of 1 mm high and 1 mm wide, was prepared in the computer with a 10 × 10 grid. The top margin was adjusted to be 0.5 mm from the top of the bur and centred from the sidewalls of the bur. This frame was used to count the diamond particles within it (Fig. 1). Figure 2 shows the tip of the bur under magnification 500 times before, and at the 10th, 20th and 30th milled crowns.
For the tungsten carbide burs, as previous studies have reported only chipping of the burs after milling, this study set out to quantify the wear. The burs were scanned before use, and the diameter (Y) and width (X) of the blades were measured (Fig. 3). It was found that the SEM scan of the burs after milling did not provide accurate quantification measurements, due to the different scales and angles when scanned, and therefore a statistical proportional method was adopted to calculate any differences due to wear. With X1 being the width after milling, the blade wear is X – X1. With Y1 being the bur diameter after milling, proportionally, X1 = X(Y1/Y) as X/Y is proportional to X1/Y1 .
The 1st, 10th, 20th and 30th crowns of both materials were filled with light-body polyvinyl siloxane impression material (3M ESPE Express. 3M, United States) and placed on the metal tooth die under a constant load of 3 kg weight, placed on the flat occlusal surface of the crown for 10 minutes. After the silicone impression material had polymerised, the excess material was removed with a scalpel blade. The material from the internal gap was taken out as one piece and weighed to calculate the overall internal fit according to the formula, (Nakamura et al., 2003): -
Thickness (internal gap) =
Where the surface area was 183.8 mm2 (calculated using FEA software) and the density was 1.29 g/ml (obtained from the manufacturer).
The 1st, 10th, 20th and 30th crowns of both materials were scanned with a 3D Measuring Laser Microscope (Olympus LEXT OLS5000, Olympus Corporation, Japan) to measure the surface roughness. On the software (OLS50-BSW, Olympus Corporation, Japan) a frame size 12 µm x 12 µm was placed on the occlusal surface of the crown to give the average surface roughness (Fig. 4). The analysis parameter used was the arithmetic mean height (Sa). It is equivalent to the arithmetic mean of the measured region on the three-dimensional display diagram when valleys have been changed to peaks by conversion to absolute values (as per Olympus resource at https://www.olympus-ims.com/en/knowledge/metrology/roughness/3d_parameter/).
Continuous variables were summarised by the mean, standard deviation, median and interquartile range. The effect of material and number of crowns milled on the internal fit (IF) and surface roughness (SR) outcomes was determined by a two-way Analysis of Variance (ANOVA) with the outcome as the dependent variable, and material and number of crowns milled as the independent variables. The interaction between material and number of crowns milled could not be assessed, as there were insufficient degrees of freedom for such an analysis. The relationship between wear and number of crowns milled for each material was explored descriptively, as were the relationships between IF, SR and wear for each material. Data analysis was carried out using SAS version 9.4 for Windows. The 5% significance level was used.