Methods: : Twelve extracted mandibular first molars with closed apex and 2-35 degree curvature where chosen. Initial images of the mesiobuccal canals of samples at 2,4,5, and 7 mm of apical length were taken by CBCT. The specimens were divided in two groups prepared with either BioRace or Mtwo system according to the manufacturer. Post instrumentation images were recorded. Residual dentinal thickness was evaluated at 2,4,5, and 7 mm of apical length
Results: Intragroup evaluations revealed no significant differences in the amount of dentinal changes (p<0.2) . In the 2mm and 4 and 5 mm of apex changes were 0.02 mm but the difference was insignificant .An estimated 0.04 mm (28.6%) dentin was removed by Biorace at 7mm of apex being more compared to Mtwo
Conclusions: No significant difference was found between residual dentin of studied groups. There was also no significant transportation in canals with curvature of 2-35 degrees. These two rotary systems can be safely used while maintaining the minimum sufficient thickness of residual dentin.