Methods: Twenty-five extracted human mandibular first molars with two separated mesial root canals were selected. The distal roots were amputated and 50 canals (25 mesiobuccal and 25 mesiolingual) were randomly assigned into two groups: group 1 - ProTaper Universal, and group 2 - Twisted File. All files were used with torque-control engine, following the manufacturer instructions, and the final instruments were F3 (group 1) and #30/.06 (group 2). Pre and postoperative cross-sections were obtained from the coronal, middle, and apical portions. Changes in dentin area and degree of canal transportation were measured and evaluated using the Bramante method and digital image software. Statistical analysis was performed using the Mann-Whitney U test (P<0.05).
Results: Preoperatively, there were no statistically significant differences regarding cross-sectional canal areas between experimental groups. In group 1 mean cross-sectional area after instrumentation had a statistically difference on cervical and middle canal thirds (p=0.040 e p=0.043). ProTaper Universal caused significantly higher canal transportation of the mesial wall in the coronal sections (p=0.039). Mean differences in distance from the center to the walls were not statistically significant in the middle and apical thirds in both groups (p>0.05).
Conclusions: The ProTaper Universal removed more dentin than the Twisted File during canal preparation on cervical and middle thirds. At coronal third, ProTaper Universal produced larger canal transportation than Twisted File.