Abstract: PRUPOSE: To compare the shaping ability of 4 nickel-titanium rotary instruments in preparation of curved root canals. METHODS: Forty extracted human maxillary first or second molars with mesiobuccal root canal curvature ranging from 20°-40° were selected. The teeth were randomly equally divided into 4 groups(n=10). Mesial root canals were separately prepared using Protaper Universal, Protaper Next, TF, and S3 nickel-titanium instruments. A series of preoperative and postoperative images were taken by Micro-CT. Mimics 17.0 software was used to analyze the following parameters: canal transportation, centering ratio values, root canal volume, volume of removed dentin, and canal/root width ratio. Data analysis was performed by using SPSS 20.0 software package. RESULTS: In terms of canal transportation after preparation at 1, 3 and 5 mm from the apex, Protaper Universal was more than the other three groups(P<0.05). The centering ratio value of Protaper Universal was significantly smaller than that of the other three groups at 1 mm from the apex(P<0.05). The amount of dentin removal was significantly different after instrumentation with the four test systems(P<0.05). Protaper Universal had the highest mean volume of removed dentin. After preparation, all root canals had a diameter that was not larger than 39% of the root diameter at the coronal and middle segments. CONCLUSIONS: Under the conditions of this study, Protaper Next, TF, S3 systems seem to be better choices than Protaper Universal system in preparing curved root canals.

Key words: Nickel-Titanium rotary instrument, Root canal preparation, Canal transportation, Curved root canal, Micro-computed tomography