Investigation of cyclic fatigue of endodontic rotary instruments used in retreatment procedure of the root canals

Aim: The aim of the study was to evaluate the effect and effectiveness of the multiuse of MicroMega Remover file on cyclic fatigue resistance during retreatment procedure of the root canals in mandibular molars. Material and method: 40 MicroMega Remover files were used in the study. In the control group, 10 new Remover files were subjected to cyclic fatigue testing using a stainless steel artificial canal with a 60° curvature angle and a 5 mm radius of curvature. Remover files (n = 10) were used to remove root canal filling material of 1 (Group I), 2 (Group II) and 3 (Group III) molars in experimental groups and retreatment procedure were performed on a total of 60 extracted teeth. In the experimental groups, cyclic fatigue tests were conducted on 30 files used in repeated root canal treatments.. The fracture time (s) of the files was recorded and the number of cycles until fracture (NCF) was calculated. The lengths of the fractured fragments (mm) were recorded and the determined samples were examined in SEM. In 1, 2 and 3 usage groups, µCT images were obtained to evaluate the amount of remaining canal filling material in the teeth. The Kruskal Wallis and Dunn test was used for statistical analysis (P < 0.05). Results: The fracture time and NCF values of Group I were found to be statistically significantly higher when compared to Groups II and III. Control Group values were found to be statistically significantly higher than Group III. When the fracture length values were compared, Group I values were found to be significantly lower than Group II and Group III. There is no statistically significant difference between the remaining canal filling amount values according to the groups Conclusion: The MicroMega Remover file can be used to remove root canal filling material of 2 molar teeth. However, due to low KAT values, it may not be recommended to use over two uses, as it may be risky. Keywords: Cyclic fatigue, static test design, MicroMega Remover, Retreatment, µBT