Abstract
BACKGROUND: Many nickel-titanium (NiTi) rotary instruments are recommended for single use, yet they are often reused for economic reasons. This study aimed to evaluate how repeated use affects the cyclic fatigue resistance and root canal wall cleanliness of the Micro-Mega Remover file. METHODS: A total of 40 new Remover files were divided into four groups based on the number of uses: Control Group (unused), Group I (1 use), Group II (2 uses), and Group III (3 uses), with 10 files in each group. Sixty extracted mandibular molars with curved canals were obturated using the single-cone technique and then randomly allocated to the experimental groups in accordance with the designated usage frequency (10 teeth for 1 use, 20 teeth for 2 uses, and 30 teeth for 3 uses). Following the retreatment procedures, all files-both from the control and experimental groups-were subjected to cyclic fatigue testing using a stainless-steel artificial canal with a 60° curvature and a 5 mm radius. The time to fracture (in seconds) was recorded, and the number of cycles to fracture (NCF) was calculated. The fractured fragment lengths (in mm) were measured, and selected samples were examined under a scanning electron microscope (SEM). Additionally, micro-computed tomography (µCT) analysis was performed to assess the volume of residual canal filling material remaining on the root canal walls after the retreatment procedure. Statistical analysis was performed using the Kruskal-Wallis and Dunn tests (p < 0.05). RESULTS: Fracture time and NCF differed significantly among the groups (p < 0.001). Group I exhibited significantly higher values than Groups II and III, while the control group also performed better than Group III (p < 0.001). No significant differences were observed between the control group and Groups I or II, nor between Groups II and III. Fragment lengths were significantly shorter in Group I compared to Groups II and III (p < 0.001). µCT analysis revealed no significant differences in residual filling volume among the groups. SEM observations demonstrated fracture initiation lines, rough surfaces, and voids consistent with cyclic fatigue. CONCLUSIONS: In this in vitro study, the cyclic fatigue resistance of Remover files was found to be similar to the control group after the first and second uses; however, a significant decrease was observed from the first to the second use. Nevertheless, the third use showed lower resistance compared to the control group and was associated with an increased risk of fracture. In addition, µCT analysis revealed residual filling material remaining on the root canal walls in all groups, with no significant differences among them.