The effect of burs wear during milling on accuracy of zirconia lithium silicate single crowns: 3-dimensional analysis

The purpose of this in-vitro study was to assess the effect of burs wear during milling on the accuracy (trueness and precision) of zirconia reinforced lithium silicate (ZLS) single crowns by using 3D software when using a chair-side 4-axis machine.MethodsA typodont maxillary right canine was scanned by Primescan and Standard Tessellation Language (STL) file was created (master case STL). Two new burs were inserted into a 4-axis milling machine and 32 zirconia lithium-silicate (ZLS) crowns (group 1- crowns 1-8, group 2- crowns 9-16, group 3- crowns 17-24 and group 4- crowns 25-32) were milled until the milling machine stopped and indicated that burs must be replaced. Each one of the 32 crowns was scanned 10 times by Primescan, creating 10 STL files for each crown. Each one of the STL files was superimposed with the master case STL file by using 3-dimentional (3D) digital software. A Kolmogorov-Smirnov test was performed followed by Linear mixed models test for comparison between the four groups regarding trueness and precision.ResultsSignificant differences were found regarding both trueness and precision. For trueness, significant differences were found between the groups as group 1 exhibited higher trueness compared to groups 3 (p = 0.026) and 4 (p = 0.019), while no significant differences were found between other groups (p > 0.05). For precision, significant differences were found between the groups as group 1 exhibited significantly higher precision compared to group 2 (p = 0.005),group 3 (p = 0.005) and group 4 (p = 0.005), while no significant differences were found between other groups (p > 0.05).ConclusionsBur wear significantly affected both trueness and precision, the first group of crowns was significantly more accurate compared to the other three groups.