The results showed that overall, all instruments may leave filling material inside the root canal. During retreatment there is a risk of instrument breakage, especially rotary instruments.

A total of 160 resin-simulated canal blocks with 20-degree or 30-degree curvature were prepared by ProFile rotary instruments, GT rotary files, and the balanced force technique with Flex-R files. Using an image analysis computer application, the pre- and postoperative images were stored and superimposed, and then the amount of material removed from the preoperative inner and outer curved walls was measured at five levels in the apical 5 mm of the canal. The time required for canal preparation, including irrigation, and the time taken to change instruments was recorded. At 1 mm from the apical end, the directions of canal transportation were most frequently toward the outer aspect of the curvature, with the only exception being the canals shaped by the balanced force technique in which more was removed from the inner aspect. The balanced force technique required more preparation time than the rotary instrumentation.

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The purpose of this study was to compare the effects of two engine-driven, nickel-titanium instrument systems with hand files in the final shape of slight and moderately curved canals. A total of 72 mesial roots of extracted human mandibular molars were divided into three groups: ProFile .04 taper, Pow-R rotary systems, and Flex-R hand-filing technique. The roots were mounted and cross-sectioned at two different horizontal levels using a modified Bramante technique. Pre- and postinstrumented cross-sectional roots were imaged, recorded, and computer analyzed. Results showed that, at the middle third, in almost all groups, there was a tendency of cutting more toward the mesial side with only one exception: Pow-R cut more to the distal side (danger zone) (p < 0.02). At the apical third, Flex-R (p < 0.03) and ProFile (0.001) transported to the mesial side (danger zone) when the curvature increased. When the three techniques were compared analyzing each side and considering the two groups of curvature, at the middle third in the moderately curved-canal group, Flex-R cut statistically more than Pow-R toward the lingual side. The other comparisons showed no statistically significant difference. When the techniques were compared in relation with the degree of curvature, in the apical third, ProFile .04 cut statistically more toward the mesial side in the moderately curved canal group than in the slightly curved canal group. The other comparisons showed no statistically significant difference. Canal preparation time was shorter with hand instrumentation (p < .05) in a few instances.