[Scan time and accuracy of full-arch scans with intraoral scanners: a comparative study on conditions of the intraoral head-simulator and the hand-held model].

Objective: To comparatively evaluate the scan time and the accuracy of maxillary full-arch scans using four intraoral scanners (IOS) on conditions of the intraoral head-simulator and the hand-held model, and to evaluate the influence of different scanning conditions on digital scan. Methods: A upper dental arch model with melamine-formaldehyde resin teeth and silica gel gingiva that could be fixed on a head simulator was scanned with an optical scanner (ATOS Core) in order to obtain the standard tessellation language dataset as reference. Intraoral scans were performed on the model fixed on the head simulator by three researchers with four IOS [A: TRIOS 3; B: CS 3600; C: CEREC Omnicam; D: iTero]. For each scanner and each researcher, six scans were performed, to obtain the datasets as the head simulator group. And another six scans with each of the four intraoral scanners were performed by each researcher on the hand-held model to obtain the STL datasets as the hand-held group. The scan time were recorded for each scan. In the Geomagic Wrap software, the digital models were trimmed with only the teeth information retained and supreimposed by best fit alignment function and compared to obtain the root mean square (RMS) values of the discrepancies by three-dimensional compare function. The test datasets of each group were compared with the reference dataset for trueness. The six test scanning datasets with the same scanner of the same researcher were cross compared for precision. Mann Whitney U test was used to statistically analyze the difference values of the scan time, trueness and precision of the same intraoral scanner between head simulator group and hand-held group. Results: Compared to the hand-held group, the scan time of A [142(82) s] and D [119(52) s], which two IOS both with handle, were longer in head simulator group [A: 98(28) s; D: 85(22) s] (P<0.01). However there were no significant differences between the two groups for scan time of IOS B and C (P>0.05). For full-arch scan accuracy (trueness and precision), there were no significant differences between the two groups of IOS A and B (P>0.05), while the trueness of C (P<0.05) and the precision of D (P<0.01) were better in head simulator group [C: 112(38) µm; D: 43(13) µm] compared to hand-held group [C: 135(47) µm; D: 53(18) µm]. However, there were no significant differences for the precision of C (P>0.05) and the trueness of D (P>0.05). Conclusions: The scan time and the accuracy of full-arch digital scans with different IOS may be effected by the scan conditions. For in vitro study of intraoral scanning, head-simulator can simulate the intraoral environment of the real patient to some extent. Meanwhile, the position of the dentist and the patient, and also the limited intraoral space during intraoral scanning are also simulated.