In this study, we showed that Mtwo removed significantly more material than ProTaper at different sellectchem levels of the curved root canals under controlled operator-related variables. Plotino et al.[13] compared the amount of dentine removed in the coronal portion of mesial roots of mandibular molars prepared using ProTaper and Mtwo instruments and found that there was no difference between the ProTaper and Mtwo groups with respect to the amount of dentine removed. The results of that study were not similar to those of the present study. Preparation symmetry is another important parameter in root canal preparations. Asymmetrical preparations may result in strip perforations or transportation of the canal, which affect the obturation procedures and thus possibly the success of the therapy.

Kuzekanani et al.[14] compared the shaping ability and cleaning effectiveness of the Mtwo and ProTaper systems in curved root canals in molar teeth and found that the Mtwo system gave a statistically smaller change in canal curvature and thus was better for maintaining the original shape of the root canal, with less transportation. The results of that study suggest that Mtwo instruments are preferable for situations where canals are curved, particularly for maxillary molars. Sch?fer et al.[4] compared the shaping ability of Mtwo instruments (using a single-length technique) with K3 and RaCe instruments (using a crown-down preparation technique) and found that canals prepared with Mtwo instruments remained better centered compared with those enlarged with K3 or RaCe instruments. Giovannone et al.

[15] compared the shaping ability of Mtwo and ProTaper instruments in simulated curved root canals in resin blocks and concluded that both instruments respected the original canal curvature, particularly in the areas at most risk of modification and they also showed good shaping ability in curved canals. Similar to the results of those previous studies, we found in this study that ProTaper GSK-3 made more symmetrical preparations in the middle portion and that no significant differences were determined in the remainder of the canal. In most of the investigations made from radiographs or composite photographic image analyses like in the present study, investigators used mathematical formulations to determine the differences between pre-operative and post-operative outer lines; however, this type of investigation may not determine the exact amount of material removed or preparation symmetry results because those analyses are two-dimensional (Mesio-Distal) and there is also another dimension (buccolingual) in samples. Thus, experiments using three-dimensional analyses may give more accurate results in the root canal preparation investigations.