AIM: To compare the apical fit in two dimensions of the first K-file versus the first Lightspeed LSX instrument binding at working length after an initial crown-down preparation.
METHODOLOGY: Twenty maxillary molars with fully developed roots and four separate root canals were selected. Canals were pre-flared with ProFile.04 taper instruments to three quarters of estimated working length. Working length was electronically determined using a size 06 K-file. Progressively larger K-files were inserted passively to working length. The first binding K-file was termed initial apical file (IAF). Then, the first binding LSX initial apical lightspeed-instrument (IAL) was determined. Teeth were scanned with and without instruments in place using micro-computed tomography. Cross-sections of the apical part of the root canals were analysed as follows: the largest and smallest diameters of the root canal and the diameter of the instrument were calculated and related to each other. Statistics were performed using Kruskal-Wallis test and Wilcoxon test. The alpha-type error was set at 0.05.
RESULTS: Initial apical file sizes ranged from 8 to 30 and were lowest in second mesiobuccal and highest in palatal canals. IAL sizes ranged from size 20 to 40. The apical large canal diameter was assessed more accurately by the LSX instruments (P < 0.0001). However, the smallest available LSX instrument (i.e. size 20) did not reach working length in 39 of 80 canals. Conclusions: Instruments with a flat widened tip were found to determine apical cross-sectional diameter better than round, tapered instruments.