Due to the introduction of dental CAD/CAM systems, dental restoration manufacturing could break from the conventional hand-crafting method, and the quality of the manufactured restorations also increased. However, many studies still indicate CAD/CAM system manufactured restorations have imprecise aspects. The imperfections were divided into digitization errors while scanning either the mouth or a stone model, and errors during milling. This study quantifies the two types of errors, and by evaluating current and novel systems, aims to provide useful information to dental clinician.
The first study evaluated the intraoral scanner’s accuracy related to digitization errors. After intraoral scanners that could directly scan the inside of the mouth were introduced to the dental industry, researchers and clinicians raised concerns about the accuracy of the intraoral scanner, and many studies had been conducted to answer those questions. However, the scanning accuracy across wide areas such as the complete arch has not been studied, and no data exists on the accuracy of the novel video method intraoral scanner. Therefore, this study conducted experiments to compare the accuracy of a video intraoral scanner and a still image scanner, and the accuracy of a complete arch scan using conventional scanning technology.
In this study, a complete arch model (ANKA-4 V CER, Frasaco, Tettnang, Germany) was scanned by a reference scanner(smartSCAN 3D, Breuckmann, GmbH, Meersburg, Germany) to obtain reference scan data. An identical model was scanned 8 times using an intraoral video scanner (CEREC Omnicam; Sirona) and an intraoral still image scanner (CEREC Bluecam; Sirona), and stone casts made from conventional impressions of the same model were scanned 8 times with a blue-light scanner as a control (Identica Blue; Medite). All scanned data were superimposed by their copies using 3-dimensional analysis software (Geomagic Verify 2015; Geomagic GmbH) to obtain a precision value, and superimposed with the reference scan data to measure trueness and evaluate accuracy. The trueness and precision of the scan data set were statistically analyzed.
During trueness analysis, when values “within the tolerance range” were excluded, omnicam and the laboratory scan values statistically differed from those of bluecam (p<.05). For precision analysis, the best root mean square(RMS), standard deviations(SD), and mean negative(-AVG) values were those of omnicam, all statistically different from those of the other groups (p<.05). The mean positive(+AVG) values for omnicam and the laboratory scan groups statistically differed from those of the bluecam group, but the values “within the tolerance range” did not statistically differ (p<.05).
The second study compared and evaluated intraoral scanner and CAD/CAM digital-work-flow manufactured zirconia and lithium disilicate crown reproducibility. In this study, a typodont model with a prepped upper first molar was set in a phantom head was scanned with a video intraoral scanner to obtain a digital impression. From the obtained digital impression, a single crown was designed, and using a milling machine, a zirconia crown and a lithium disilicate crown were each manufactured (n = 12). All manufactured crowns were digitized and saved with an optical reference scanner.
The digitized data was superimposed with 3D inspection software to quantitatively obtain the reproducibility of a ceramic crown, and visual differences were confirmed with a color map. Areas with a large visual difference were observed with a digital microscope. The root mean square (RMS) values of ceramic crown group were statistically analyzed with Student’s t-test (α = 0.05).
According to the result of the second study, the RMS value of the lithium disilicate crown was 29.2 (4.1) ㎛ for the outer surface and 17.6 (5.5) ㎛ for the inner surface, and for the zirconia crown, 18.6 (2.0) ㎛ for the outer surface and 20.6 (5.1) ㎛ for the inner surfaces. The reproducibility of the zirconia and lithium disilicate crown had statistically significant difference at the outer surface (p < 0.001) but had not significant difference at the inner surface (p = 0.234). Visual analysis revealed the outer surface of the lithium disilicate crown had over contouring at the buccal surface, and the inner surface had under contouring at the marginal surface. On the outer surface of the zirconia crown, both over and under contouring existed at the buccal surface, and on the inner surface, under contouring at the marginal surface.
From the two studies, not only the intraoral scanner accuracy, but crown reproducibility during the digital workflow of intraoral scanning and milling could also be measured and evaluated. The video intraoral scanner was much better at digitizing complete arches than the still image intraoral scanner, but could not completely replace the conventional method, and moreover, it could be confirmed that manufacturing restorations using a digital workflow process resulted in better reproducibility for a zirconia single crown than a lithium disilicate single crown.