Studies on Cutting Aspect of Root Canal Orifice Enlargement with Nickel-titanium Rotary Instruments

Abstract

Nickel-titanium orifice openers were designed to prepare the coronal and middle third of the canal. Three different types of rotary nickel-titanium endodontic instruments, QuantecFlare® K³ OrificeOpener® and Tac Endo Flare®, were selected for investigation. In order to investigate the effects of the nickel-titanium instrument type, the insertion speed during cutting, and the use of sodium hypochlorite during cutting, we continuously measured the torque at the enlargement of the root canal orifice using a specially designed computer-controlled torque testing device. Cutting instruments were directly immersed in a plastic dish containing 10% sodium hypochlorite solution or 10% sodium hypochlorite paste for 60 minutes, and rinsed under running distilled water. The rotational speed was fixed at 300 rpm. Instruments were then inserted to a depth of 8 mm into standardized models of the resin straight root canal model with different insertion speeds (0.01 mm/sec and 0.05 mm/sec). The cutting efficacy was analyzed based on the torque-insertion distance and the calculation of the variation in torque (ΔT). Morphological observations of the cross-sectional profiles were also conducted. The following conclusions were obtained: 1. The three types of instruments had different cross sections. This suggests the possibility that the blade structure affects cutting. 2. The integral value ΔT of the cutting torque and insertion distance are affected by the type of cutting instrument and decreased significantly in the order of Tac Endo Flare®, K³ OrificeOpener® and Quantec Flare®. 3. ΔT was affected by the insertion speed and was significantly greater for an insertion speed of 0.05 mm/sec than for 0.01 mm/sec. 4. ΔT tended to decrease in the order of the nonimmersed group, the sodium hypochlorite solution gorup and the sodium hypochlorite paste group.