Effect of different preparation designs of minimally invasive occlusal onlays on the accuracy of different intraoral scanners: An in vitro study

Purpose: The aim of this in vitro study was to assess and compare three different preparation designs of minimally invasive occlusal onlays on the trueness and precision of three different intraoral scanners under two different scanning conditions.Materials and Methods: Three maxillary premolars were prepared in three different designs and divided accordingly into three groups, Group 1: Anatomical (n = 60), Group 2: Flat (n = 60), and Group 3: Ferrule (n = 60). The samples were then further divided into subgroups according to scanners as subgroup A: Medit i500 (n = 20), subgroup B: 3Shape TRIOS 4 (n = 20), and subgroup C: Cerec Primescan (n = 20). Last, the samples were further divided according to scanning conditions: Division i: As prepared (n = 10) and Division ii: Sprayed - scan spray (n = 10). An industrial 3D scanner was used to obtain the reference STL files. Accuracy was assessed in terms of trueness and precision and recorded in terms of root mean square in micrometers. Numerical data were explored for normality using Shapiro-Wilk test and were analyzed using 3-way ANOVA followed by Tukey's post hoc test.Results: Regarding trueness, 3-way ANOVA showed that all tested variables had a significant effect on trueness. Significant interactions were found between the different variables (p < 0.001). For preparation design the highest value was found in ferrule preparation (27.88 +/- 7.11), followed by flat preparation (22.99 +/- 7.56), while the lowest value was found in anatomical preparation (18.83 +/- 5.71) (p < 0.001). For scanner type, the highest value was found in Primescan (25.36 +/- 10.66), followed by TRIOS 4 (22.75 +/- 5.98), while the lowest value was found in Medit i500 (21.59 +/- 5.03) (p < 0.001). As for the scanning condition, sprayed samples (26.54 +/- 8.24) had a significantly higher value than non-sprayed samples (19.93 +/- 5.53) (p < 0.001). Regarding precision, both preparation design and scanner type had a significant effect on precision. Scanning conditions had no significant effect. There was a significant interaction between the three tested variables (p = 0.012).Conclusions: Anatomical preparation of minimally invasive occlusal onlays produced the most accurate scans. Within the tested preparation designs, Medit i500 and 3Shape TRIOS 4 have better trueness than Cerec Primescan. Cerec Primescan is more precise than 3Shape TRIOS 4 and Medit i500 Scan spray application causes a higher deviation in the trueness of the tested intraoral scanners while it does not affect their precision.