Shaping ability and cleaning effectiveness of Mtwo versus coated and uncoated EasyShape instruments in severely curved root canals of extracted teeth

Abstract

To compare the cleaning effectiveness and shaping ability of Mtwo, PVD-coated and uncoated EasyShape nickel-titanium rotary instruments during the preparation of curved root canals in extracted human teeth. A total of 60 root canals of mandibular and maxillary molars with curvatures ranging between 25° and 35° were divided into three groups of 20 canals. Based on radiographs taken prior to instrumentation with the initial instrument inserted into the canal, the groups were balanced with respect to the angle and the radius of canal curvature. Canals were prepared using a low-torque control motor according the single-length technique. Using pre- and post-instrumentation radiographs, straightening of the canal curvatures was determined with a computer image analysis program. Preparation time, changes of working length and instrument failures were also recorded. These data were analysed statistically using ANOVA and Student-Newman-Keuls test. The amounts of debris and smear layer were quantified on the basis of a numerical evaluation scale. The data established for scoring the debris and the smear layer were recorded separately and analysed statistically using the Kruskal-Wallis test. During preparation, one coated and two uncoated EasyShape files fractured. Completely clean root canals were never observed. All instruments maintained the original canal curvature well with no significant differences between the different instruments (P > 0.05). Instrumentation with Mtwo files was significantly faster than with the two other instruments (P < 0.05). For debris removal Mtwo instruments achieved significantly better results (P < 0.001) than the two EasyShape instruments. The use of coated EasyShape files resulted in significantly less debris compared to instrumentation with uncoated EasyShape instruments (P < 0.05). The results for remaining smear layer were similar and not significantly different for the coronal, middle and apical third of the canals (P > 0.05) but in summary significantly less smear layer was observed following instrumentation with Mtwo (P < 0.01). Under the conditions of this study, all instruments maintained the original canal curvature well. The use of Mtwo instruments resulted in good canal cleanliness. PVD-coating of EasyShape instruments had no impact on their shaping ability but improved their cleaning effectiveness.