Three-dimensional morphological analysis of 67 primary mandibular first molars based on CBCT reconstructions

Abstract Background Despite previous anatomical studies on PMFMs, detailed classification and comprehensive measurements remain limited, and the association between crown and root morphology has not been systematically investigated. This study aims to reconstruct three-dimensional anatomical models of PMFMs from intact CBCT images obtained from children in Beijing, China, to characterize variations in crown and root morphology and their relationships, and to conduct detailed quantitative and statistical analyses based on refined classifications. Methods Three-dimensional (3D) anatomical models of 67 intact PMFMs were reconstructed from CBCT scans using a semi-automatic threshold-based region segmentation method. Key morphological parameters of the crown and root were measured. Samples were primarily classified based on the ratio of mesiodistal to buccolingual crown diameter (R) and the number of roots (N), respectively, and their association was explored. A secondary classification was conducted by incorporating the number of root canals (n). For subgroups with sufficient sample sizes, detailed morphological parameters were measured and compared among types. Results Two crown variants were identified. R > 1 was observed in 59 teeth (88.1%), with a mean value of 1.11 ± 0.06, while R < 1 was observed in 8 teeth (11.9%), with a mean of 0.94 ± 0.03. A significant difference was found between the R values ( p < 0.05). Two root number variants were identified. Fifty-eight teeth (86.6%) had two mesiodistally aligned roots, all with R > 1. Nine teeth (13.4%) had three roots, in which 8 (11.9%) had R < 1; and 1 (1.5%) had R > 1. A significant negative correlation was found between R and N ( r = -0.61, p < 0.05), and ROC curve analysis confirmed that R = 1.00 was the optimal cutoff for distinguishing two-rooted from three-rooted PMFMs (AUC = 1.000). Besides, a secondary classification identified seven types, among which the mesiobuccal canal was consistently the longest ( p < 0.05). Conclusion PMFMs exhibit considerable morphological variations. The ratio of mesiodistal to buccolingual crown diameter (R) is a useful predictor of root number, and the mesiobuccal canal is significantly longer than the other canals. These features may aid clinicians in preoperative planning.