It is difficult to obtain complete primary teeth because of the physiological or pathological absorption of deciduous roots. CBCT provides a feasible method by which to study the root canal morphology of deciduous teeth. In this study, we obtained complete 3D structural data of deciduous teeth through CBCT. Different from previous micro-CT or nano-computed tomography (nano-CT) in vitro studies[13, 16], the teeth included in this study were all normal teeth or teeth with caries not exceeding the superficial dentin, with healthy pulp-dentin complexes in vivo. Such samples can reflect the morphology of the teeth under the physiological state and are of great significance for obtaining thorough knowledge of the normal anatomy of primary teeth.
Indices for root canal ageing should not only reflect changes in the root canal morphology on CBCT but also have clinical significance to serve as a reference for related treatment. The canal diameter is directly related to the file size . Because of the irregular cross-sectional shape of the canal, measuring only the maximum and minimum diameter of the root canal cannot clarify the relationship between the inner diameter of the root canal and the file. In this study, the diameter of the canal segment of interest was measured at all positions, and the proportion of the root canal with a diameter less than 0.15 mm, i.e., the R.15, was calculated. The cut-off value of 0.15 mm was chosen because the #15 K-file is commonly used as the initial manual file to prepare the root canals of deciduous molars. This observation variable can reflect the possibility of the file entering the canal and the proportion of the lateral wall area of the region of interest that can be touched by the #15 K-file on plane D. This measurement index can provide more information than the diameter of the root canal in a certain direction. The cross-sectional area of the canal can directly show changes in the canal without being affected by the cross-sectional canal shape, which is a common index used in root canal measurement[18, 19].
In this study, the variable R.15 of MB roots increased with age, and the cross-sectional area of MB root canals on plane D decreased with age, indicating a decrease in the canal diameter. Similar results were found in the orifice of the mesial root canals of mandibular primary molars (P < 0.01). These findings indicate that as children age, the canal portion with an inner diameter larger than 0.15 mm decreases, and the #15 K-file becomes less likely to enter the root canal on plane D. Furthermore, we observed that most MB canal cross-sections were oval or flattened on plane D (Fig. 3). The presence of an oval canal and a smaller canal diameter increase the difficulty of mechanical and chemical canal instrumentation, resulting in larger areas of the dentin wall being untouched. However, the R.15 of DB and P canals was not found to correlate with age in this study. This is because most DB canal diameters were smaller than 0.15 mm on plane D at these young ages. In contrast to DB canals, for P canals, nearly all diameters on plane D were larger than 0.15 mm for all ages. However, the area of DB and P canals on plane D decreased significantly with age, reflecting age-related changes in the canals.
Changes in the anatomy of the lower half of the root canal were reflected by the variables LC/LR and Ndis. The results showed that LC/LR decreased and Ndis increased significantly with advancing age. Secondary dentin deposition causes root canal shrinkage, which can even block small-diameter canals, resulting in canal shortening. However, canal disappearance was usually observed to be incomplete on CBCT, and in some canals, the minor lumen could still be observed under the deposited dentin. This may be caused by different rates of dentin deposition and small differences in the inner diameter of the root canal. Because of the lower resolution, these discontinuous canals may show no root lumen on two-dimensional clinical radiographies, which may affect the accuracy of root canal working length confirmation and both lead to incomplete root canal preparation and increase the risk of leaving residual infectious substances.
Because the lengths of the three roots of the maxillary first deciduous molars were significantly different, we chose to divide the roots into three equal parts to observe the changes in the canals with age, as in previous studies. During the experiment, it was found that for children over 5 years old, most of the DB root canal lumen and the majority of the MB root canal lumen could not be seen on the plane positioned at the midpoint of the apical two-thirds of the root length. Therefore, the plane where the midpoint of the coronal two-thirds of the root length lies was chosen as the measurement plane, which was named plane D.
In this study, four feasible observation indices were successfully developed for careful observation of the root canal morphology of deciduous teeth. It is hoped that more clinicians will participate in relevant work in the future to revise and improve these indices so that they can more accurately reflect the morphological characteristics of the root canal morphology of deciduous teeth and improve our understanding of the root canal morphology of deciduous teeth.
There are some limitations to this study. First, for the study of age-related changes, obtaining CBCT scans of the same child at different ages is the best approach. However, children are sensitive to radiation, so CBCT cannot be performed only for scientific research. Therefore, it is more practical to choose scans from children of different age groups for comparison. Second, due to the strict indications for CBCT in young children, although the teeth included met the sample size requirements, all children were from one clinical centre, and the results of some indices may be affected by individual differences. However, the changes with age were still obvious. The final research results need to be verified by studies with larger samples at multiple clinical centres. Third, due to the limitations of the resolution of CBCT, only the main root canal could be studied, and root canals smaller than the voxel size could not be identified.