Investigation of the effects of different working lengths on crack formation during the root canal preparation with current NiTi rotary file systems

In our study, we observed cracks on the dentin when the root canals were instrumented at different working lengths with ProTaper Next (Dentsply, Maillefer, Ballagues, İsviçre), Mtwo (VDW, Munich, Germany), Hyflex CM (Coltene, Allstatten, Switzerland) NiTi rotary files and Mani Flare NiTi hand files (Mani Inc., Tochigi Japan) which have different geometric shapes, different tapers and different metarological features. The aim of our study was to observe the effects of these variations on the cracks on the dentin. In our study, 260 extracted human mandibular incisor teeth were used. The teeth were randomly selected in 12 experimental groups (n: 20) and 2 control groups (n: 10). Preparation of the root canals of the teeth has been completed with NiTi rotary systems and NiTi hand files with apical width of 0.30 mm at 3 different working lengths. The roots were sectioned horizontally at 3, 6 and 9 mm from the apex. Taken sections and apical surfaces of teeth were examined by a stereomicroscope for crack detection. Although there was no statistically significant difference when the study lengths were evaluated, more cracks were observed in AF + 1 groups than AF and AF-1 groups. Although no statistically significant difference was observed when NiTi systems were evaluated, it was observed that the most cracks were in the MT group (10.6% in the sections, 8,3 %the apical surface). PTN (5.6% in sections, 5% in apical surface), HF (3.9% in sections, 1.7% in Apical surface), Mani Flare NiTi hand file(1.1% in sections, 1.7% in apical surface) were followed respectively. As a result of our study, It is observed that preparate with NiTi rotary file systems, at 1 mm short from the apical foramen and when preparate apical end 1 mm, using NiTi hand files or non-traditional NiTi rotary file systems are the most reliable method for crack formation. Key words: Dentinal cracks, NiTi rotary systems, different working lengths