Objective: To analyze effect of fractured file removal from the middle third root canal on root fracture resistance using finite element analysis, which provides a theoretical basis for clinical prognosis evaluation. Methods: Two finite-element models were established, the fractured file removal model (fractured file located in the middle third of root canals, followed by ultrasonic file removal and root canal preparation) and the control model (root canal preparation only), and compressive displacement dependencies on compressive force was computed and compared with experimental data for validation. The validated finite-element models were used to analyze the stress distribution differences during the initiation, propagation and completion of the crack between fractured file removal specimen and control one. Results: The critical breaking force of the fractured file removal specimen was 406 N, and the finite element simulation result was 396 N. The critical breaking force of the control specimen was 502 N, and the finite element simulation result was 483 N. The position of crack initiation in the finite element simulation was basically consistent with that in the experiment. The experimental data of compressive test and the results of finite-element computation were in agreement, thus validating the finite-element model. In the process of continuous pressure, the stress distribution of the control root is relatively uniform, and the location of crack initiation and the direction of propagation have a certain unpredictability. Compared with the control root, the stress concentration on the root with fracture file removal was obvious, especially on edges, and the number of cracks are much more. Because of the thinner radicular wall, the crack propagation rate is faster too. Therefore, the overall root fracture resistant is decreased obviously. Conclusions: During the fractured file removal procedure, amount of dentine removed should be minimized, and the edges and corners which caused by fractured file removal should be shaped to smooth in order to reduce the stress concentration and prevent the root from fracture.
目的： 采用有限元法分析超声法取根管中部分离根管锉对牙根抗力的影响，为临床评估预后提供依据。 方法： 构建取分离根管锉样本（根管锉分离于根管中部，超声法取出后根管预备）及对照样本（仅进行根管预备）的三维有限元模型，在有限元模型上模拟牙根抗压载荷实验过程，将有限元计算结果与实验结果进行对比，验证有限元模型的准确性。在验证后的有限元模型上，比较分析取分离根管锉样本及对照样本的根管壁裂纹产生、扩展、裂开时的应力变化差异。 结果： 取分离根管锉样本，牙根最大抗压载荷为406 N，有限元模拟结果为396 N；对照样本牙根最大抗压载荷为502 N，有限元模拟结果为483 N。有限元仿真的裂纹萌生位置与实验基本一致。有限元模拟结果与实验测试结果一致，验证了有限元模型的有效性。对照样本应力分布均匀，裂纹萌生、扩展的方向均有一定的不可预测性。取分离根管锉样本的根管棱角处应力集中严重，裂纹产生条数多；根管壁薄，裂纹扩展速度快，牙根整体抗折力明显下降。 结论： 取分离根管锉后应尽量减少对牙本质的切削，将根管壁棱角处修整圆滑以减少应力集中，从而减少牙根折裂的风险。.
Keywords: Finite element analysis; Fractured file; Ultrasonic technique.