A comparative study of seven instruments in shaping the root canal in vitro

The objective of this study was to evaluate the shaping characteristics of various root canal instruments using human teeth. A total of 420 extracted human roots were embedded in resin blocks. The embedded roots were divided into three groups, i.e. roots with (i) straight, (ii) apically curved, and (iii) whole-length curved canals according to the Schneider's angle and the length of the radius of arc fitting the curvature of root canals. Each of the three groups containing 140 roots were randomly divided into seven subgroups prior to preparation. The canals were prepared manually with traditional and flexible instruments, engine driven Racer-type, Giro-type, randomly vibrating instruments and with sonic and ultrasonic instruments. The shortcomings of the seven preparation methods were assessed by the superimposition of projected radiographs taken in bucco-lingual and mesio-distal views before and after the preparation. The prepared straight canals showed a high percentage (75.7%) of apical asymmetry. Coronal transposition of the apical stop was graded with higher scores in all canal forms prepared with Racer-type and Giro-type instruments, which could have occurred through packing of debris towards the apical constriction. The majority of the prepared curved canals were asymmetrical in shape. The location of the aberrations depended on the original shape of the canal but the method of shaping also had a decisive effect on the post-operative form of the canal. Large differences were found between the incidence of elbow (11.7-40.0%) and zip (75.7-80.0%). K-Flex files produced minor canal aberrations and significantly less asymmetry (P < 0.05) than the conventional hand instruments. The MM 1400 handpiece and ultrasonic instruments were associated with less aberration and significantly less asymmetry than the other instruments tested. The Excalibur appeared to be superior to the conventional hand instruments in straight canals, but its shaping characteristics were similar to conventional instruments in curved canals. Under the conditions of this study, canal shaping with Cavi Endo, MM 1400 and K-Flex files appeared to be superior to that achieved with conventional hand instruments Excalibur, Intra Lux Endo Kopf 3LDSY, and 3LD instruments.