# Study of Root Canal Length Estimations by 3D Spatial Reproduction with Stereoscopic Vision
## 4 Discussion  
In this study, we developed an application, which is capable of viewing computer-generated 3D images, for root canal length measurement using a naked-eye stereoscopic display (SRD). We performed both objective evaluations of measurement accuracy and subjective evaluations, such as eye fatigue. For the objective evaluation, we compared the measurement values and measurement times for four different tooth models with different root canal morphologies. There was no significant difference in measurements between the SRD and the 2D device. The Bland-Altman analysis showed that the SRD had a lower standard deviation (SD) of measurements compared to the 2D device. Previous studies have reported measurement accuracy errors of 0.02-0.59 mm for dental arch models when using SRD, indicating the possibility of an accurate 3D computer-generated stereoscopic model display [22]. In our study, the measurement range (maximum value minus minimum value) for the four different dental models ranged from 0.16 to 0.62 mm. We also evaluated the measurement method by measuring the four different tooth models with different morphologies. The #44 tooth model was found to be the easiest to measure. When measuring tooth models, it is necessary to set the first reference point on the occlusal surface and the second reference point at the root canal apex. The #44 tooth model has one cusp and one root canal, which makes its morphology particularly straightforward, and the difficulty of measurement is low. However, the Bland-Altman analysis showed that the #44 tooth model had the highest number of measurements that deviated from the limits of agreement (LOA). This is believed to be due to the specific measurement method of the 2D device and the morphology of the #44 tooth model. Typically, when measuring a 3D model with a 2D device, all three axis planes (X, Y, and Z) must be displayed simultaneously, and the central axis is set  
\begin{table}
\begin{tabular}{l c c} \hline \hline  & **r** & **\(\mathbf{p}\)** \\ \hline Age group 1 [FOOTNOTE:1]Footnote 1: The variables included age group (20 \(\leqslant\) 0, 30 \(\leqslant\) 1, 40 \(\leqslant\) and over: 2) 1, gender (female; 0, male: 1) 2, 2D device usage history (first use: 0, several times: 1, about once a month: 2, about once a week: 3) 3, and eye fatigue (feel tired: 1, do not feel tired at all: 10) 4. [FOOTNOTE:1]Footnote 1: The variables included age group (20 \(\leqslant\) 0, 30 \(\leqslant\) 1, 40 \(\leqslant\) and over: 2) 1, gender (female; 0, male: 1) 2, 2D device usage history (first use: 0, several times: 1, about once a month: 2, about once a week: 3) 3, and eye fatigue (feel tired: 1, do not feel tired at all: 10) 4.  
on each axis plane. Since this study was limited to root canal length measurements, the measurement on the y-axis, which is the long axis, becomes important. Therefore, if the y-axis measurement is performed without adjusting the x and z axes from the condition captured by the cone beam computed tomography (CBCT), it becomes impossible to set the maximum cusp point on the occlusal surface. Tooth Model #44 has a cusp and a root canal; thus, it was initially measured without adjusting the axes, which may have resulted in misalignment due to the examiner's assumptions when setting the reference points. As a result, using the SRD for measurements created a sense that the tooth model being measured was right in front of the examiner, thus reducing the measurement errors that are caused by examiner assumptions. On the other hand, the #26 tooth model (Figure 1b) presented measurement difficulties. To measure the maximum long axis during the root canal length setting in the #26 tooth model, it was necessary to select one of the three cusps as the first reference point and one of the three root canals as the second reference point, which made it more difficult. Therefore, with the 2D unit, all axis planes had to be adjusted from the initial setup. However, only one measurement deviated from the limits of agreement (LOA) according to Bland-Altman analysis.