# Abstract
## 4 Discussion  
In this study, digital data of dental models used in conventional training, obtained by an intraoral scanner, were loaded, and abutment formation was performed on the spatial reality display (SRD), which enables the naked-eye stereoscopic viewing of 3D-CG dental models, thereby improving the cost-effectiveness of abutment formation practice. This study examined whether performing abutment formation on the spatial reality display (SRD), which enables 3D-CG tooth models to be viewed stereoscopically with the naked eye, improves the cost-effectiveness of abutment formation practice and provides sufficient learning effects. Objective evaluations of the tooth formation and subjective evaluations, such as the feeling of using SRD, were conducted. For the objective evaluation, the upper-left first molar of a conventional mannequin-based tooth model was scanned to compare the ideal abutment tooth morphology with those created by the participants. The discrepancy and over-formation rates were lower with the SRD than with the 2D display and statistically significant differences were observed, indicating that the use of SRD resulted in lower discrepancies in the exemplar abutment tooth morphology. This indicates that it is easier to reproduce the ideal abutment tooth morphology via simulations using the SRD. The SRD makes it easy to obtain the depth information by changing the observation angle, which is important for obtaining the shapes of 3D objects, and thus facilitating the understanding of the 3D position of the tooth model and cutting instruments during the shaping operation. This may have led to the high accuracy of the tooth formation, demonstrating the significance of SRD for preprothetic treatment. Previous studies have shown that SRDs can be used to accurately represent dental arch models [46]. Additionally, for root canal length measurements, the accuracy errors ranged from 0.16 to 0.62 mm [40]. Abutment tooth morphology is imperative in prosthetic pretreatment, and a taper of 2-5\({}^{\circ}\) is ideal for prosthetic retention [47; 48]. In the proposed application, one side of the spatial resolution of the tooth model was divided by 2\({}^{\circ}\). One side of one of the resolved squares was approximately 0.21 mm. Based on the resolution, an ideal taper of 2-5\({}^{\circ}\) can be reproduced from a height of 4.2-10.5 mm. In addition, the actual formation of the abutment tooth requires a thickness that takes into consideration the material properties of the final prosthetic device (full crown) and a formation that does not damage the blood vessels and nerves (pulp) that exist inside the tooth. In other words, about 2 mm of formation is necessary to ensure the thickness of the fully encapsulated crown [49]. It is also necessary not to remove more than about 6 mm [50; 51] from the occlusal surface to avoid damaging the pulp. The resolution of this application is sufficient to reproduce the results. Therefore, we believe that the accuracy of abutment tooth formation in SRD can be ensured. The automatic scoring function calculates the quantitative endpoints as volumes based on the voxel volume of the exemplars. One percent of the quantitative endpoint is approximately 3.6 mm\({}^{3}\). The actual tooth preparation of a fully coated molar crown has a cutting volume of 250-330 mm\({}^{3}\)[52], which is a sufficient percentage of the quantitative evaluation item to the second decimal place to maintain sufficient accuracy to perform evaluation. In addition, since visual feedback in terms of over- or under-formation can be provided by the color scheme, it is easy to evaluate the formation together with the quantitative evaluation items. We used the first molar as it has a more complex abutment tooth morphology, and a similar validation is required for other tooth types; however, because sufficient significance was obtained for the validation of the first molar in this study, we believe that the same results can be obtained for other teeth.