Abstract: BACKGROUND: Previous studies on root canal filling with thermoplasticized gutta-percha and single-point technique have mainly focused on the density of root canal filling, while research on the root’s resistance to fracture is relatively scarce. Understanding the stress distribution and crack characteristics of the root under different filling methods is of great significance for improving the success rate of root canal treatment. 
OBJECTIVE: To compare the maximum compressive load strength of the root of the maxillary second premolar after root canal filling with thermoplasticized gutta-percha vertical compaction and iRoot SP single-point technique, and to analyze the stress concentration areas, crack features, and overall fracture resistance of the root. 
METHODS: (1) The discarded maxillary second premolars extracted for orthodontic reduction were selected and filled with thermoplasticized gutta-percha vertical compaction and iRoot SP single-point technique respectively. The in vitro teeth were subjected to vertical loading tests using an electronic universal testing machine to detect the location of root cracks and the maximum yield force and maximum fracture resistance of the root. (2) The maxillary second premolar original data were obtained by cone beam CT scanning, and three-dimensional reconstruction was performed using Mimics 21.0 and Geomagic 2021 software. SolidWorks 2021 was used for model assembly. The filling models made by thermoplasticized gutta-percha vertical compaction and iRoot SP single-point technique were established respectively. ANSYS Workbench software was used for mechanical analysis to analyze the maximum principal stress and stress concentration site of the root.
RESULTS AND CONCLUSION: (1) Microscopic observation showed that cracks occurred in the buccal and lingual directions of the roots of both groups. The maximum yield force of the root in the thermoplasticized gutta-percha vertical compaction group was 336.4 N, and the fracture resistance was 1 124.6 N. In the iRoot SP single-point technique group, the yield force was 468.8 N, and the fracture resistance was 1 263.7 N. (2) Finite element analysis showed that with the increasing pressure value, the maximum principal stress values of the roots of the two groups showed an upward trend. The maximum principal stress value of the roots of the iRoot SP single-point technique group was always higher than that of the thermoplasticized gutta-percha vertical compaction group. The root stress of the thermoplasticized gutta-percha vertical compaction group was concentrated around the root canal filling material, and the root stress of the iRoot SP single-point technique group was mainly distributed around the gutta-percha tip. In the case of flattened root canals, the root stress concentration area and crack initiation position were mainly located in the buccal and lingual directions. The crack initiation position predicted by three-dimensional finite element simulation was highly consistent with the actual root crack position observed in the in vitro experiment, confirming the validity of the finite element model. (3) The study indicates that for the flattened root canals of maxillary second premolars, the iRoot SP single-point technique shows significant advantages in root fracture resistance and stress distribution.

Key words: flattened root canal, single-point technique, thermoplasticized gutta-percha vertical compaction technique, root canal filling, finite element analysis, root longitudinal fracture