Accuracy of digital duplication scanning methods for complete dentures
A master cast was selected based on the American College of Prosthodontists (ACP) Prosthodontics diagnostics index type A classification of residual ridge morphology. The selected master cast was scanned using a desktop scanner (7 Series; Institute Straumann AG). The scanned master cast was used to digitally design a complete denture using CAD software (Dental Studio; 3Shape). The designed denture was imported to open‐source software (Meshmixer; Autodesk Inc.) and segmented into four segments (denture extension, dentition, intaglio, and combined) as reference files. The digitally designed complete denture was exported as an STL file and 3D printed using a digital light processing (DLP) 3D printer (Asiga Max; Asiga, Sydney, Australia) with light‐polymerizing resin (Crown and Bridge, DENTCA, Figure ). The sample size and power analysis for this study were calculated based on a previous study with a similar approach. A sample size of 10 per method would allow 80% power to detect an effect size of 1.325 between methods, based on a two‐sample t ‐test calculation at a two‐sided 5% significance level. The printed complete denture was digitized utilizing different scanning methods, and four study groups ( n = 10/group) were included in this study. Group A: Cone beam computed tomography (CBCT) (Planmeca Viso G7, Planmeca, Helsinki Finland), Group B: Desktop scanner (7 Series; Institute Straumann AG), Group C: Trios intraoral scanner (3 shape Trios 4, Copenhagen, Denmark), and Group D: Virtuo Vivo intraoral scanner (Virtuo Vivo intraoral scanner; Straumann AG). For group A, the denture was scanned 10 times by a CBCT scanner (Planmeca Viso G7, Planmeca, Helsinki Finland). The scanning protocol included 100 kV, 40 mAs, fields of view (FOV) of 80 ×  80 mm, and a voxel size of 139 μm. The DICOM files were reconstructed using open‐source software (InVesalius 3.1, CTI, Brazil) and exported as STL files. For group B, the denture was scanned 10 times by a desktop laser scanner (7 Series; Institute Straumann AG). The scanning protocol involved scanning the cameo surface of the denture first, followed by the intaglio surface. The two scan surfaces were then combined into a single STL file using open‐source CAD software (Meshmixer; Autodesk Inc). The scans were exported as STL files. For groups C and D, the denture was scanned 10 times using a Trios intraoral scanner (3 shape Trios 4, Copenhagen, Denmark) and Virtuo Vivo intraoral scanner (Virtuo Vivo intraoral scanner; Straumann AG), respectively. The scanning protocol for groups C and D involved separately scanning the intaglio, cameo, and border of the complete denture. In the first scan, the intaglio surface of the denture was scanned, and the scan extended beyond the denture border in a wavy motion from posterior to anterior (Figure ). In the second scan, the cameo surface was scanned beyond the denture border, starting from the right maxillary tuberosity to the left maxillary tuberosity buccally and continuing to the occlusal surface from the left maxillary tuberosity to the right maxillary tuberosity. The palate was captured in a wavy motion. Subsequently, the denture border was scanned using the interocclusal record scan in the scanning workflow. The three scans were then superimposed and merged using the overlap of the border with the assistance of open‐source CAD software (Meshmixer; Autodesk Inc). The scans were exported as STL files. The STL files gathered from all four groups were segmented and divided into intaglio, denture extension, dentition, and the combined surfaces in measuring the trueness and precision of each method. The trueness and precision were evaluated by superimposing the four surfaces STL files (intaglio, denture extension, dentition, and the combined file) from the scanned surfaces of the denture to the corresponding reference STL files of the digitally fabricated denture, resulting in four reference files and 160 test files. All the digital file comparisons were made in surface matching software (Geomagic design X; 3D system, Rock Hill, SC) using the best‐fit alignment method. The data collected from the same surface were grouped and analyzed. Deviations between the test and the corresponding reference STL file were shown in mean, standard deviation (SD), and root mean square (RMS). The RMS value represented the absolute value of the dimensional difference between the study samples and the original digitally designed denture. Moreover, the software color mapping feature was utilized to illustrate the digital file accuracy of the scanning method on the four surfaces. F ‐tests were used to compare the groups for differences in within‐group standard deviations among the 10 samples. One‐way ANOVAs were used to compare the groups for mean differences in deviations from the original dentures in RMS while accounting for unequal group variances. Two‐sided 5% significance levels were used for all tests. All statistical analyses were performed using SAS version 9.4 (SAS Institute, Inc., Cary, NC, USA).   Descriptive statistics, including mean and standard deviation (SD), are presented in Table . The RMS values for each group and surface are summarized in a boxplot diagram in Figure . The results indicated that the CBCT group had the highest RMS values, while the DS and TIO groups had the lowest RMS values among all groups. To examine trueness, one‐way ANOVA was used for mean differences in deviations from the reference file in RMS. The means of RMS used for the trueness, are shown in Table . Regarding scanning accuracy of the entire denture, the CBCT group showed the highest RMS (0.249 ± 0.020 mm) and lowest trueness compared to the DS (0.124 ± 0.014 mm p < 0.001), TIO (0.131 ± 0.006 mm p < 0.001), and VVIO (0.227 ± 0.020 mm p = 0.017) groups, while DS and TIO showed significantly smaller RMS than VVIO. For the trueness of dentition, denture extension, and intaglio surfaces, the CBCT group also showed the highest mean RMS and lowest trueness among all groups ( p < 0.001). In contrast, the DS and TIO had smaller mean RMS and higher trueness among all groups in all surfaces ( p < 0.001, except VVIO in intaglio surface p > 0.05). To evaluate the precision, F ‐tests were used for differences within‐group standard deviations among 10 samples. Table also shows the standard deviations of the RMS values used to measure precision. When measuring the combined surfaces, TIO had significantly lower variability of RMS within each sample compared to the compared to DS ( p = 0.013), CBCT ( p = 0.001), and VVIO ( p < 0.001) group. This suggests that TIO had the highest level of precision among all groups. For dentition and denture extension surfaces, the DS and TIO groups had similar within‐group variability of RMS ( p > 0.05) and lower (more precise) values than the CBCT and VVIO groups ( p < 0.001). For the intaglio surface, CBCT and TIO groups exhibited similar within‐group variability of RMS ( p = 0.693) and lower (more precise) values than the DS group ( p = 0.037). TIO showed lower within‐group variability of RMS and higher precision in the intaglio surface compared to the VVIO group ( p = 0.022). Color maps of the surface matching differences for each surface are shown in Figure . Areas in blue indicate negative discrepancies, and areas in yellow/red indicate positive discrepancies when comparing the scan samples with the digitally fabricated denture. The area in green indicates surface matching within ±0.10 mm. In the combined surfaces, the CBCT group showed more overall negative discrepancies. The VVIO group showed more positive discrepancies (red) in mid‐palatal in the combined surface. In the intaglio surfaces, TIO showed more green in the alveolar ridge.   In the present study, the digital file accuracy of a duplicated complete denture using different scanning methods (CBCT, 7 series desktop scanner, Trios intraoral scanner, and Virtue Vivo intraoral scanner) was investigated. The null hypothesis was rejected suggesting that different scanning techniques affect the trueness and precision of duplicated dentures. CBCT showed the lowest trueness of all groups in the examined surfaces. Moreover, CBCT and VVIO were less precise than DS and TIO in combined, dentition, and denture extension surfaces. On the other hand, TIO had the highest precision among all groups in the combined surface. Digital denture duplication is usually done by using a desktop scanner. This could be due to the higher resolution of the desktop scanner, the object being fixed, natural light being blocked to increase the performance of the scanning, and a larger camera than the intraoral scanner to reduce the number of image superimposition errors. , In general, desktop scanners are more often used by dental laboratories instead of dental offices. As a result, it is often necessary to send dentures to the lab technician to use the desktop scanner to duplicate the denture. Other possible methods of duplicating dentures include using either CBCT or intraoral scanners, which are now widely available in dental offices. With the improvement of CBCT and intraoral scanners, denture scanning by CBCT and intraoral scanners has been reported in the literature. , , When using an intraoral scanner to duplicate a denture, the final accuracy of the datasets could also be influenced by variations in the image acquisition technology used by different optical scanners. Three types of optical scanners were included in the present study; Trios 4 IOS uses confocal microscopy to construct 3D data from a number of two‐dimensional images. The desktop scanner (7 Series) is based on blue laser triangulation technology with free movement of axes, while multiscan imaging technology is used in Virtuo Vivo IOS. Different optical scanners’ scanning accuracy in dental implants and tooth preparation has been reported in several studies. Diker et al. reported that Trios 4 IOS resulted in better trueness compared to Virtuo Vivo IOS in complete arch scanning. Baghani et al. and Chen et al. showed that DS was more accurate than IOS for full arch span. , Çakmak compared the accuracy of in vivo scans of Virtuo Vivo IOS and Trios 3 IOS to 7 Series DS for full arch implants placed in an edentulous mandible. The results showed that the distanced deviation in DS was the highest whereas those in Trios IOS were the lowest. However, to the best of the author's knowledge, there are insufficient studies that compare the accuracy of duplicating complete dentures with IOS and DS. One study by Matsuda et al. performed the geometric accuracy of imaging of a complete denture form using the DS, and the handheld scanner. The results of the study suggested that handheld scanners had lower accuracy than DS. Although the scanners used in the study were different, the result of the present study partially agrees with the finding from Matsuda et al. DS scanner showed higher trueness than VVIO, but it was not significantly different from TIO. However, it should be noted that Matsuda et al. only compared the whole scan surface and did not examine the individual intaglio and polished surfaces or the dentition surface and border extension, which were specifically analyzed in this study. The findings from the current study suggest that DS still showed higher trueness in dentition surface and border extension. The reason for the contrasting outcomes in the present study and Matsuda's research might be attributed to different scanners, denture materials, and experimental procedures. Moreover, not all scanners are capable of scanning both the polished and intaglio surfaces in one single scan. To ensure a standardized scanning process, the present study combined the intaglio surface scan and polished surface scan using CAD software to generate all the duplicate denture STL files. However, the merging of the two scans may have caused some distortion, which could have contributed to the discrepancies observed. Fully guided implant surgery treatment planning for an edentulous patient usually involves a CBCT scan and a dual scan protocol. In dual scan protocol, the denture is scanned with CBCT and merged with the patient's CBCT data set to allow prosthetic‐driven guided implant placement. It was reported that to obtain the maximum accuracy from the CBCT scan, each voxel size has an optimal segmentation threshold. Thus, it would be difficult to standardize the segmentation threshold for all scans. The smoothing procedure of the DICOM file was reported to reduce the size of the actual image by up to 12%. To improve denture duplication accuracy, Guilherme et al. proposed using IOS to scan the existing denture to improve the quality of the guide by reducing CBCT artifacts associated with the dual scan. However, there is no consensus regarding whether IOS or DS is superior in accuracy of duplicating dentures. Al‐Rimawi et al. compared the trueness of a dry human mandible scanned using Trios 3 IOS and four different CBCT machines. The results of the study showed that CBCT‐derived 3D models showed better trueness than IOS. In contrast to the previous study, Michelinakis et al. reported that IOS created superior trueness compared to CBCT when scanning dental casts. The findings of the present study agree with the study conducted by Elkhadem et al. It was found that the CBCT group had higher RMS values compared to the intraoral scanner group. However, the results of this study were inconsistent with studies conducted by Chen et al. and Matsuda et al., which compared the trueness of complete denture duplication using CBCT and desktop scanners. According to their results, CBCT and desktop scanners were found to have similar trueness values. , Chen et al also concluded that CBCT results in comparable trueness across dentition, denture extension, and intaglio surfaced. On the contrary, the present study reported that CBCT showed significantly lower trueness across all four surfaces (dentition, denture extension, intaglio, and combined). These inconsistencies in both studies could be attributed to segmentation threshold, scanner hardware and software differences, and scanning patterns, as well as different analysis software. Color maps in the present study illustrate the discrepancies in the combined surface of the dentition and denture base extension. The CBCT group exhibited more negative discrepancies in the combined surface at the dentition and denture base extension. Both CBCT and VVIO groups showed negative discrepancies at the denture extension border. These findings suggest that negative discrepancies at the dentition may lead to a reduction in the vertical dimension of occlusion. In contrast, discrepancies at the denture extension borders could impact the denture border seal and result in reduced denture retention. In the intaglio surface, all groups showed negative discrepancies at the posterior palate which could affect the posterior palatal seal of the denture. In addition, DS and VVIO groups showed more negative discrepancies extending to the alveolar ridge, which may indicate the need for a denture reline. CBCT group showed more positive discrepancies at the crest of the alveolar ridge, which could indicate the need for a denture adjustment. The limitation of this study is that the ridge morphology that was selected does not take excessive undercuts and long denture extensions into consideration. Protocols followed for denture duplication using scanners were mostly relatable to the ideal case scenarios. Further studies are required to discuss the same in complete dentures with different morphologies and scanning protocols. Besides additive technology, different complete denture fabrication techniques can also be considered in future studies. Although the results of the presented study added evidence on multiple digital denture duplication scanners, with the rapid advancement of scanners and scanning methods, more studies are needed to provide practitioners with clear guidance to achieve optimum patient care.   The 7 Series desktop scanner and Trios 4 intraoral scanner can duplicate dentures in higher trueness than CBCT and the Virtuo Vivo intraoral scanner. The Trios 4 intraoral scanner creates more precise results in the combined surfaces than other scanning methods, while the desktop scanner and Trios intraoral scanner are more precise in denture extension surface.   